Having used debian stable for decades now, I upgraded my servers to buster in July.
PHPBB 3.2.7 kept running without any problem.
Today I wanted to update phpbb to 3.2.8 and failed due to php 7.3 used in buster. I crawled the web a little and found:
who offers a variety of php packages for debian/ubuntu. I installed 7.2 did the update and switched back to 7.3-fpm.
No issues so far, but I will backup regularly.
Would greatly appreciated to have a phpbb version that will keep track with debian stable php packages, though. You are not used to get hit by a "you use software that is to new" problem as a debian stable user
HTH somebody else hit by the same issue.