phpBB 3.2 still uses jQuery 1.11.0 which has been released in 2014. It contains a number of security vulnerabilities: https://www.cvedetails.com/vulnerabilit ... query.html
and is less efficient performance-wise compared to the latest version.
I read that some of the phpBB's current code is incompatible with the new versions of jQuery, however if this project continues to move forward it will require to rework this code sooner or later. The longer it is delayed, the harder it will become to update it.