There was some discussion on the PR about the approach of defaulting to asking the server side to handle all the client-side 404s. The Discourse core members are concerned that it will make the existing 404 routes worse, and @eviltrout wants to solve it on a case-by-case basis by building up the SERVER_SIDE_ONLY whitelist.
I’ve made a PR to start that off
https://github.com/discourse/discourse/pull/4660