After installing a new version of Eclipse, the create XHTML file option wasn't where I remembered it. I had already installed JBoss Tools. Under this version (the March 2019 Eclipse with JBoss Tools added), the option is found under File -> New -> Other -> JBoss Tools Web -> XHTML Page
.
After you select File -> New -> Other
, the wizard that shows includes a "type filter text" box. If you enter XHTML there, it will show the relevant option and where it is found. If I had noticed that first, that would have been easier than Googling and would have produced the correct answer. Presumably that will continue to work even after future updates so long as there is some XHTML wizard available.