Some of phpBB's responsive elements are controlled by JS, specifically it's things like:
- Link labels for Notifications / PMs / contact us / the team / members / delete cookies being hidden and replaced by an icon
- Navlinks (such as FAQ) are removed and collapse into the "Quick links" toolbar
- Breadcrumbs are condensed
- Tabs are condensed
- forumlist columns are hidden and replaced with topic / post counts below forum description
I would suspect the reason for these being handled by JS as opposed to CSS would be primarily because:
1. These areas are often template-event heavy, and are therefore variable in nature. CSS media queries are outstanding for responsive design, but only if you reliably know the content you're working with. If you have a lot of extensions, linklists can often get quite long, and defining a fixed CSS responsive breakpoint just isn't going to work.
2. Ancient framework. Prosilver was built in 2006(?) and is still used as the primary framework to this day. Responsive design wasn't a consideration back then, which is why there are lots of hacky solutions present today.
I really wouldn't anticipate any improvements on the CLS front in the phpBB 3.x.x development branch, but 4.0.x will come with brand new and modern front-end technologies, which you can see here: viewtopic.php?t=2485796