YaBB is still under active development. I know, because I'm one of the developers on the project ;-)
We're focusing on the next major version which is a rewrite using much better coding standards.
SMF used to be called YaBB SE, which was a direct port of YaBB to PHP. Development hasn't stopped on the PHP version of YaBB, because there never was a PHP version of YaBB. YaBB SE turned into SMF, mainly because they felt that people were prejudiced against YaBB, simply because it's Perl.
If your web host is pushing for a non-Perl forum, ask them for their reasons, because we've (the dev team) seen--and refuted--them all.