They still distribute runtime(s). Proton runs inside one of those runtimes. You're talking about 1.0 version, 2.0 was based on debian 10, and 3.0 is based on debian 11.

It still has some assumptions about host system, though, but that's a problem for those who package steam. For example, my non-FHS NixOS provides everything required, and it works out-of-the-box.