My understanding is that Sympl won’t upgrade to v12 until Debian does the same.
Meanwhile, Debian will continue to supply security fixes for PHP7.4 so it’s safe to use, but won’t have the new functionality of PHP8.
There is no official schedule, but past experience suggests mid 2023 for Debian 12.
This seems to happen with every Debian stable release: PHP security support runs out before the new Debian release comes out. It’s not a real problem because of the continuing Debian security support.
Not directly I think, but sympl-phpmyadmin depends on phpmyadmin, and sympl-webmail depends on roundcube, which both use PHP.