In most cases the game warns you with the exception of certain bigger changes (i.e. sex-changing characters which is kinda obvious). I know of a few that kinda apply. I don't know how far along you are so I'll try to be vague.
One I do know of though is with Azzyran. If you do the corrupt end to her questline it becomes impossible to recruit her as a companion but she can still be interacted with in the hive.
There is also a unique weapon called the Dawnsword acquired during Ryn's questline, specifically
The Demon of Ice High Ground
(thank you Ballin for correcting)
I don't remember exactly how it works but there are two versions of it and which one you get depends on your choices and require different levels of corruption to use. Each version also has its own achievement tied to it when acquiring it.
Another one that is kinda along the same line is a certain genie character can provide unique boons but depending on your choices you can lose out on those.
In most cases the game warns you with the exception of certain bigger changes (i.e. sex-changing characters which is kinda obvious). I know of a few that kinda apply. I don't know how far along you are so I'll try to be vague.
One I do know of though is with Azzyran. If you do the corrupt end to her questline it becomes impossible to recruit her as a companion but she can still be interacted with in the hive.
There is also a unique weapon called the Dawnsword acquired during Ryn's questline, specifically
The Demon of Ice
at the end of Act I
. I don't remember exactly how it works but there are two versions of it and which one you get depends on your choices and require different levels of corruption to use. Each version also has its own achievement tied to it when acquiring it.
Another one that is kinda along the same line is a certain genie character can provide unique boons but depending on your choices you can lose out on those.
The dawn sword doesn't appear in "the demon of ice" quest but it does have the crown of winter (only after getting the two pieces in the specter of wyld quest).
The dawn sword is its own quest called "high ground" given to you by sanders after completing the winter wolf quest which only needs you to complete the winter city portion of in her foot steps and land of milky honey to get
It's very rare but there some examples where content can be permanently missed.
You can skip the farmer freeholds quest if you go to pupperidge farms first.
You can miss the Crown of Winter if you don't get the shard and the broken crown from specter of the wyld first.
Vaush is permanently missable if you complete right of conquest before ever encountering him.
Azami's old forest encounter is missable if you defeat the alrune first.
These are the only examples I can think of that don't involve a PC failing a quest or intentionally skipping something.
The dawn sword doesn't appear in "the demon of ice" quest but it does have the crown of winter (only after getting the two pieces in the specter of wyld quest).
The dawn sword is its own quest called "high ground" given to you by sanders after completing the winter wolf quest which only needs you to complete the winter city portion of in her foot steps and land of milky honey to get
This is honestly one gameplay decision I never understood. Why da fuck are you almost always punished (mechanically) if you play a submissive character.
You play with Domrona? Sorry you now lose Alissa content. Just wanna enjoy sucking some dick? Better be careful, you could be getting bad ended....
There is no equivalant for dominant pc´s.
Why do this? Who wants that? Why is my brain now wired to associate dom = potential danger? IDK. And I hate it.
This is a much more complicated issue, which I also wanted to open a post on. I think it has roots in a mix of gamist tradition and immature vision of sexual relationships: associating "winning the game" with dominance, and being defeated with submission.
So for example, usually to access the submissive options you have to be "defeated", with loss of EXP, gold or content. Although in the case of bad ends I suppose for some it is a reward.
The Fungal Mass can be missed in that Boreal Druid zombie complex. I like to imagine Lumia cares for them if you don't.
Oh, and I guess I should mention you get the Lucky Coin for beating them.
There is a missable scene at the start of the game, actually. At least I never found a second way to trigger it
You get a scene for drinking the blue potion in the prologue.
Defeating the Alarune causes the Forest Kitsune in the Old Forest to move to the den. If you don't help them before that, (and also don't have the Amulet of Union) you won't be able to avoid the fight with them during the Den of Foxes quest.