I'm mostly HTML5 at this point and have been for some time. The only exception I would make is things (say, Standard Ebooks) that haven't made the jump yet so their linters freak out if you don't do things exactly as they expect.
For the record, anyone who writes `<br />` as `<br>` is...misinformed.
It's not our (SE's) choice to use XHTML in ebooks, it's part of the epub spec. So we merely expect valid XHTML per the epub spec, and valid XHTML requires self-closing tags.
For the record, anyone who writes `<br />` as `<br>` is...misinformed.