What is the advantage of PPAs over an apt repo that users place in /etc/apt/sources.list.d/someexternalapp? Is it having a central site that users can search for packages? Basically a social portal?
Having an automated per-user publishing pipeline for packages is a non trivial endeavour.
These days you can get close with github-actions style pieplines and releases, but the PPA system has always been a more complete platform in terms of dependency management.
You can get close with some debian tooling (im a fan of sbuild), but it's some overhead to deal with.