Are there standard versions of `sed` on which `-E` is unsupported?

GNU first added undocumented support for -E just to be compatible with BSD syntax, and the source included the comment

/* Undocumented, for compatibility with BSD sed. */

But in 2013 that was removed in this commit with the log message

Modify documentation to note sed "-E" option, now in POSIX, for EREs.

and the commit references a defect tracker for POSIX at this page that marks as accepted adding the -E flag to the sed arguments

It doesn't seem to have made it into the latest POSIX spec (sed specific part) though, but I guess it's coming.

The place to look is at systems which are certified:

OSX has it, though whether it's BSD- or GNU-first was not part of the question.

Related