"Usually?" What are the unusual circumstances? Will I need to care about it at an inconvenient time two years later?

I mean, yes, I can probably guess some things and end up with something that works with the current version of the library, but why would I choose this library to build on top of if I'm just guessing?

The code cannot possibly answer the question of what changes the developers may make to the project in releases they consider backwards-compatible. My question is entirely about the API, not about the code itself.