SphinxSearch 3.x changes it's license and the opensource 2.x version is not present in the release of Debian 11 bullseye.
As mention here:
viewtopic.php?p=15647081#p15647081
I think it is a good idea to add Manticore Search to phpbb search backend.
manticoresearch:
https://github.com/manticoresoftware/manticoresearch
For anyone who wants to use manticore to replace sphinx:
[GUIDE] Setup manticore search backend for phpbb