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When I'm playing an RPG, or analyzing its design, there are several things I'm constantly watching for.
One of these things is irreversible events: occurrences that block off alternative actions I might have taken or prevent access to alternative content I might have enjoyed.
I don't like them. Mostly.
What's An Irreversible Event?
Imagine a single path between two areas. The path leads to a collapsed ledge. You jump down, then find that the path above is too far up to reach again.
Another example: imagine crossing a bridge over a wide chasm. Every place you've visited so far can be reached again if you stop, turn around, and go back. Instead, you cross the bridge, but as you reach the far side, the bridge behind you crumbles and falls away. You're now stuck on this side; there is no mechanical means in the game by which you can return to the previous side. None of those areas can be reached now.
Yet another example: you're creeping through the dark when you hear a chitinous rustling somewhere ahead of you. You don't want to make a light, but you have only one potion of Night Vision remaining that you wanted to save for later. You decide it's more important to stay hidden, so you quaff the last Night Vision potion and carefully evade the nest of spiders now visible. Later, you can hear multiple skeletons clattering in the dark ahead. To avoid running into them, you must make a light. The skeletons see you and attack, forcing you to enter combat instead of enjoying being sneaky.
One more example: your character is having a conversation with another character. You're playing your character as cool and smart, trying to keep future dialog options open with this other character. But in clicking on a speech option, your finger slips and you choose an aggressively physical response option. The character you were talking to gets angry at you, walks away, and will no longer talk to you for the rest of the game. Because the developer provides no way to reload from a recent save, you're faced with the choice of either losing hours of play or missing out on what might be a fun interaction with an interesting character. (In a visual novel you might just replay the whole game to see the "missed" content, but let's say here that this interaction was just one small but interesting part of a much larger RPG.)
The Squishing of Opportunity Space
What irreversible events such as these have in common is that they foreclose options. Where prior to an irreversible event there might have been 100 different things the player could choose to do, after such an event there might be only 50 possible actions, or 10, or in extreme cases only one. The opportunity space for fun has been shrunk.
Certainly not everyone will find this objectionable. But there is a subset of gamers who will object, because an important source of fun for them in games is in being able to demonstrate their cleverness. They have the most fun when a game enables and encourages surprising-but-effective choices or creative combinations, or allows them to use their pattern-detection gift to perceive a particularly good option out of a mass of possibilities. When instead a game is designed such that there's only one (obvious) solution to each problem, or in which a mere handful of worthwhile mechanics exist that can't be combined, there's little opportunity for cleverness. These players will know they're being marched through the developer's predetermined story.
That's not a mentally interesting game. It may be an exciting experience; or it may be a moving experience; or it may be a low-stress way to pass some time. But it's not an most appealing kind of fun for gamers who prefer interactive entertainment that exercises their brains as well as their hands and hearts. For some gamers, less choice is less fun. And irreversible events decrease choice.
When Are Fewer Options OK?
There are some practical exceptions to this. As noted above, not every game has to emphasize thoughtful fun. It's fine that there are games offering other kinds of fun; the gamers who like having lots of choices aren't the only kind of gamer. If your target player prefers exciting or emotional or low-engagement fun, then keeping the possibility space limited to deliver a highly focused play experience may be the right design approach.
It also might be possible to offer too much undifferentiated choice, such as from a combinatorial system that lets players bang together hundreds of inputs to yield millions of possible outputs. (Although here I would argue the true design flaw is the "undifferentiated" aspect.) And of course implementing consumables such as food or potions is almost always a reasonable feature. Consumption as a one-way process does foreclose options, but as an economic phenomenon -- a value choice -- this can be a useful way of increasing the consequentiality of player decisions, which is itself a worthwhile design goal.
Foreclosing on personal interaction options with characters is trickier, but I can appreciate the argument that "this is how conversations with people in real-time actually work." However, I'm not sure this is a strong argument for an RPG -- at least, not until better AI allows NPCs to respond plausibly to surprising inputs from the player, rather than just following pre-written branching logic no matter how narratively bizarre and mechanically undesirable those character reactions may seem from a player's perspective.
More importantly, story beats in general will usually need to be one-way. It would be extremely interesting to find a game that's designed -- not just by implementing save/reload, but through a first-class gameplay mechanic -- to allow the player to backtrack to any previous story events to explore different alternatives! Short of this, it's pretty reasonable that once the player has triggered a plot point, then short of reloading a savegame there's no going back from that story event or its directly related consequences in the game world.
(This points out an interesting difference in the kinds of irreversible events: events in time, and events in space. Irreversible events that lock off currently existing physical or systemic spaces are IMO a Bad Thing for exploratory fun. But irreversible events that occur sequentially in time are necessarily, in virtually every case, desirable as one-way processes because that's usually how story works.)
Player Ingenuity Is Scary... And Worth Supporting
Overall, then, with the exception of story beats it seems to me that a pattern of implementing or allowing irreversible events -- consequences that reduce opportunities for player choice -- is a big red flag for exploration-oriented gamers. It's a warning that the developer is not actually committed to supporting player freedom and creativity in overcoming challenges and exploring the game world's content, but instead wants to tightly control the player experience.
Conversely, it is a thrill to discover a game that visibly minimizes irreversible events, that I can see has taken pains to ensure that players can almost always backtrack to any location or non-story state. When I play a game that's careful not to foreclose my options, I know it's made by a developer who is confident in the high-level flow of the game, who is genuinely delighted when I discover a new way to interact with the game world, and who respects my fun-finding autonomy.
If you've built a world that's worth exploring, why not allow and maybe even reward backtracking?