Steam as flatpak works, thank devs. It has some issues and it doesn't seem to be up-to-date but it works.
One game gave me a disk error, something like "Either you don't have enough space on the disk or the disk is corrupted.". Plus, when launching Steam from the terminal one can see a lot of these messages:
"process 4778: arguments to dbus_connection_ref() were incorrect, assertion "connection->generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2656.
This is normally a bug in some application using the D-Bus library."
Issue report: https://github.com/ValveSoftware/steam-for-linux/issues/5146
For the unaware, you can install Steam as flatpak by first installing flatpak: https://kaosx.us/docs/flatpak/
Then adding the 'flathub' remote:
$ sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
And finally installing Steam:
$ sudo flatpak install flathub com.valvesoftware.Steam
To run it, just:
$ flatpak run com.valvesoftware.Steam
Edit: the disk error message could be my disk dying after all...
Edit 2: aparently my disk is fine. Noticed /tmp getting full during instalations... :/