But then why is it working for everyone else?

That’s the question here :) Flatpak it self can’t be the issue because other Flatpaks work. My system can’t be the issue because my system works. And yes, the Steam Flatpak on it’s own also can’t be the issue because then more people would be affected and I would find something during web research.

How about the package manager version?

There isn't really a Steam version in the repos but only in multilib. So I enabled it and installed steam from there. It also crashes. The only difference is that there are some fontconfig things it cannot understand.

GetWin32Stats: display was not open yet, good
GetWin32Stats: display was not open yet, good
steamwebhelper.sh[19338]: Runtime for steamwebhelper: defaulting to /home/dirk/.local/share/Steam/ubuntu12_64/steam-runtime-heavy
steamwebhelper.sh[19338]: glibc >= 2.34, partially disabling sandbox until CEF supports clone3()
Fontconfig warning: line 5: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 4: unknown element "description"
Fontconfig error: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 72: non-double matrix element
Fontconfig error: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 72: non-double matrix element
Fontconfig warning: "/etc/fonts/conf.d/10-scale-bitmap-fonts.conf", line 80: saw unknown, expected number
Fontconfig warning: "/etc/fonts/conf.d/10-sub-pixel-rgb.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/10-yes-antialias.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/11-lcdfilter-default.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/20-unhint-small-vera.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/30-metric-aliases.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/40-nonlatin.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/45-generic.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/45-latin.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/48-spacing.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/49-sansserif.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/51-local.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/60-generic.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/60-latin.conf", line 4: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/65-nonlatin.conf", line 4: unknown element "description"
CAppInfoCacheReadFromDiskThread took 0 milliseconds to initialize
src/steamUI/steamuisharedjscontroller.cpp (540) : Failed creating offscreen shared JS context
src/steamUI/steamuisharedjscontroller.cpp (540) : Fatal assert; application exiting
08/19 01:26:26 Init: Installing breakpad exception handler for appid(steam)/version(1690583737)/tid(19319)
assert_20230819012626_30.dmp[19438]: Uploading dump (out-of-process)
/tmp/dumps/assert_20230819012626_30.dmp
dirk ~ $
assert_20230819012626_30.dmp[19438]: Finished uploading minidump (out-of-process): success = yes
assert_20230819012626_30.dmp[19438]: response: CrashID=bp-175add07-33af-421a-a870-828612230818
assert_20230819012626_30.dmp[19438]: file ''/tmp/dumps/assert_20230819012626_30.dmp'', upload yes: ''CrashID=bp-175add07-33af-421a-a870-828612230818''

So it's Steam in combination with my system. This time the mentioned dump is there but it does not contain any information I could use/understand.

This is what all the people who had to remove xdg Desktop portal use. How could they remove it if it’s a dependency of flatpak?

When reinstalling Flatpak it asked for xdg-desktop-portal-impl to be installed and offered multiple packages that provide it.

dirk ~ $ pacman -Ss xdg-desktop-portal | grep installed
extra/xdg-desktop-portal 1.16.0-3 [installed]
extra/xdg-desktop-portal-gtk 1.14.1-1 [installed]

It also worked earlier during testing (at least one time it started up and let me log in but subsequently it crashed on any of the further tries) so all dependencies are satisfied.