F-Droid (default repo) offers old versions. Good luck downgrading apps from Play Store.

It also makes it easy to find open-source apps. Generally they also provide more info useful to me as a regular user - manuals and changelogs. Open-source app changelogs often show far more info, even library version changes. Meanwhile the average Play Store app changelog is “small improvements, better ad experience”

Play Store needs to implement version archive and license filters. Oh, and easier beta access. Currently the registration takes extra time, and can only be done with the original account the app was installed with.