Is anyone aware of an exhaustive list of warnings that ca65 can emit?

I've never seen one. CC65 has an option --list-warnings which could be responsive, but ca65 doesn't have something similar. You can search the source code. They're spread throughout the individual files. Search for Warning, Internal, Error, Fatal.

Good find; there are a lot less warnings than other C compilers I'm used to. I opened https://github.com/cc65/cc65/issues/1757

I looked at the source code for that option and it's not clear to me whether it lists all warnings or just a list of warning types. The latter is implied in the code, but that seems trivial since there seem to be less than a dozen types.

I know from a search of the source code there are about 1000 or so calls to the Warning, Internal, Error, Fatal routines, but many of these represent the same type of warning, just in different source files.