IIRC, you can still add those repos. The PPA system just makes it a little easier to add them.

it's one of those things that will probably work most of the time

PPAs are built against Ubuntu packages as dependencies - which usually but not always match Debian ones. Easy way to get subtly broken programs

Can you explain how better? A link to an example maybe

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.

Agreed. I'd love to switch to Debian, but the lack of PPAs makes it difficult.

Also Debian is always much further behind than Ubuntu on new packages.