I see a lot of snippets that do:
"Is it portable" is not quite the same question as "is it POSIX".
export var=value is specified by POSIX, but it is not portable if you target the wider group of all (current and legacy) Bourne-like shells.
I don't know if there are any systems currently being sold by a vendor that don't allow this syntax. But again, "systems currently being sold" and "systems currently running" are different questions. We'd need some actual users of AIX, HP-UX, Solaris, etc. to stop by and tell us...
You may find a script using the two-step "assign then export" for any of these reasons:
exprfor arithmetic, and the mystical construction