There are way more than fit on the chart, so I'm mostly focusing on the ones that are fully libre (including data), and that I have some familiarity with.
Going by their Mastodon account, seems they were erroneously detected as "from a US-sanctioned region" and it took too long for said error to be resolved, so they just made the switch.
Yeah, only thing I can think of is the few banking apps that don't have web versions.
I was lucky enough to have all my banking and 2FA apps work perfectly on GrapheneOS. The only app that gave me a significant amount of trouble was iClicker, which my school uses for attendance. That was fixed by enabling Google Play location services, and there was a (fairly expensive) alternative anyway.
I did have to buy a new phone to use Graphene, because I got my previous one as part of a carrier's cell plan, and it had a locked BIOS. Though I think the purchase was worth it, and just moving my SIM card from one device to another was enough to get it working.
OC please steal
Debian needs a better installer. It'd be awesome if it had something more akin to Fedora/RHEL's Anaconda, or even just made Calamares the default (so long as it didn't install every single locale available like their live inages currently do).
Most regular distros are good enough for gaming. The only issue you're likely to run into is with graphics drivers (I recommend going for AMD graphics on that build), and the availability of certain software in certain formats (gaming software is more likely to be available for Debian or Ubuntu based distros).
If you like the Steam Deck's desktop mode, you might enjoy another distro with the same desktop environment (KDE Plasma). I'm partial to KDE Neon, a snappy Ubuntu LTS spin with all of the latest KDE software.
Just as long as it isn't a .rar.