There is a difference between locking a core mechanic and additional content. No duh, give the player all the gameplay tools. That shit shouldn’t be missable and nobody is arguing that but you should have missable side quest and loot. Or one way story choices.

I like sign posts. It's fine to me that once you leave Lothering you can't do the quests there anymore. I don't really like the "you did a thing, and now you can't do these other unrelated things" thing. That's the "You didn't go to the river to meet the wizard" example. That's just annoying and arbitrary. That's not a choice. There's no way to intuit what's going to happen. That's not interesting.

And yes, yes, if I go to the subway now versus an hour from now I'll see different people and have a slightly different experience, but that's not very interesting to me. I would be annoyed if a game was like "On day 2, if you go to the subway between 7:58 and 8:03 you meet a wizard and can join his academy" without any foreshadowing or clues. Maybe you like that kind of thing. I'm probably going to use a guide (on a second playthrough if it seems like a game I'll replay, or right away if it seems like I'm not going to).

you are robbing players of potential experiences

You could easily make the argument that you're robbing players of potential experiences by putting a cool thing in an easily missable spot. I don't think that's a very compelling argument.

That’s how life works and should be present in games, especially in rpgs.

This is a terrible argument. Life is terrible and full of nonsense that does not need to be represented in games. You don't need to model swallowing water wrong and having a coughing fit in games, but that's in life all the time. The goal is not to make a completely accurate model of life. The goal is often to make a fun experience.

Anyway. I think we probably disagree in degree rather than in form. A lot of things I don't really mind are missable- like kirk's armor in DS1. But some things are just really annoying, like the wizard-at-the-river example. I don't know if I can come up with an objective rubric for when I go "no, this is bullshit let me see that guide."