I use phpBB 3.0.14 on all four of my public phpBB boards. They have been patched to work with PHP 7.4, and they each have completely custom themes (they aren't styles, phpBB devs!!!) written by me.
I have the latest versions of each phpBB3 branch running on a test site. It's astonishing how much more memory 3.3 uses compared to 3.0, and how much slower it is. (And for fun, download the 3.0.14 release package and compare the number of files in it versus the new 3.3.4 release package.)