The board.php file contains 24 <br>.

It seems the all the other files use <br /> (442).

Is phpBB suppose to be fully HTML5 or is it using XHTML on some pages?

If its only suppose to be HTML5 everywhere then perhaps only use <br>.

But <br> and <br /> is valid in HTML5 but personally i perfer the simplest.

XHTML uses <br />.

https://area51.phpbb.com/docs/dev/3.2.x/language/validation.html says:

Additional <a href="">, <strong>, <em>, <u> and <br /> are allowed.

Perhaps that should be updated.