When I choose "Save as…" from 2021 (25.3.1) it appears that I can only save is as a 2020 file, not 2021. Why? I am wondering
The file format hasn't changed. Probably just an oversight where someone forgot to update the UI string in the list.
Not an oversight I think. You'll see that although Illustrator has over 20 versions, there are only 12 formats. It's a good thing when it doesn't change on a new version, because sharing files is simpler.
if I definitely want to overwrite the file, although Illustrator believes it is not changed (or if I want to change the setting of "pdf compatible"), "save as" to current version and same name was the best way to do.
So I think I will miss something
"Current version" means "current version of file format". Currently, that would be "Illustrator 2020 file format". Or so it seems to me.