No issues
Windowed or borderless modes freeze
Works perfectly out of the box. No tinkering needed.
Display Server: X11 - ProtonVersion: 7.0-6 - Tested with an Xbox One Controller.
No issues found
Font sizes can be increased with "Settings" > "Interface" > "UI Scale" > "Full-Bodied".
If Valve were to review this game, I'm guessing it would be Playable due to font size (which can be changed), this game otherwise works great on Steam Deck.
Internet acces is required on first lauch, becomes optional after enabling download mode in the settings.
PROTON_USE_WINED3D11=1 %command%
Check the guide: https://steamcommunity.com/sharedfiles/filedetails/?id=2682039000

7.0rc6-GE-1 GloriousEggroll

Game works near-perfectly if you disable Steam Input and use at least Proton-6.21-GE-2 as the compatibility tool.
Proton-6.21-GE-2 GloriousEggroll
Proton-6.21-GE-2
Switching between fullscreen and borderless maximised caused 3/4 of the window to be cut off, though since the game uses a custom cursor (controllable by mouse or controller) I was able to use it to click the button that was out of view and return to fullscreen.
With Steam Input enabled for the game, connecting a controller mid-game caused the Steam overlay to lock up for a few seconds, after which both the raw controller and the virtual Steam controller were detected and controlled individual input slots. Disabling Steam Input for the game fixed input issues.
Main menu slowed down to 30 or 40 frames per second, expected 60 given my hardware. In-game levels and editor were stable 60fps.

Proton-6.21-GE-2 GloriousEggroll
Proton GE is required. It's a custom fork of Proton and requires manual installation.
See this Steam guide to learn how it works.

Does not even start with Proton 6.21-GE-2
It fails with the error below:
Win32 function failed: HRESULT: 0x887a0002
Call: at line 230 in file \GraphicsDisplayM.cpp

The game has quite low framerate but it's running on Linux at least. It will probably be better when bleeding edge Proton got stabilized.
Proton-6.21-GE-2 GloriousEggroll
I downloaded bleeding edge Proton.
Framerate in fullscreen or windowed is just too slow, makes it unplayable.
Finally, the game seems to be working fine with Linux. I did use the GloriousEggroll version of proton.
I had to play the game windowed (not borderless) otherwise there were some stuttering. Might have something to do with Wayland.

proton-ge-6.20-1 GloriousEggroll

All online features seem to be working, on a newer proton-GE version

Crashes while trying to connect to Rumpus server
I ran the game with "Proton Experimental", which is right after Proton 6 at the time of writing this.
The game launches with "PROTON_USE_WINED3D11=1 %command%" launch options. However, as it shows up initial menu, it tries to connect to the server, and there it fails, saying:
___________________________________________
############################################################################################
ERROR in
action number 1
of Step Event0
for object o_rumpus_item_downloader:
Data structure with index does not exist.############################################################################################
gml_Object_o_rumpus_item_downloader_Step_0 (line 22)
Does not even launch

The main appeal of game is create levels and play custom levels, but the game crashes as it needs to connect to server to do that.
Multiplayer don't work. After level 3 I guess, the game tries to connect to server to download user levels and it crashes.
I liked it which made the whole "not have access to the main selling point" even more sucky.
The game crashed without disabling esync.
The game will NOT connect to the servers and you will CRASH if you try. You can delete files to be able to play again, just don't do what I did and lose all your saves. ".local/share/Steam/steamapps/compatdata/792710/pfx/drive_c/users/steamuser/Local\ Settings/Application Data/PlatformerBuilder"
Looks like you can play it although there is this connection problem.
Gane tries to connect to server, but regularily fails. After some time and automatic automatic attempts, connection looks like beig establisheh. Unclear if everything works but at least it created me a user.

Didn't get past fading into the main menu
The game launches, shows the developer splash screens and attempts to fade into the main menu. Like other reports, it crashes when trying to load anything from the game's servers to download levels. In the brief time the game ran, sound worked fine and the graphics worked identically to Windows at the right resolution.
Online resource doesn't work
Online play does not work.
Trying to download a map from online will crash the game.

No online levels accessible.
Can't access online games, that's kinda the most important part.

The whole online part doesn't work, it seems like a network related issue since the solo works, however if you login to their servers, the game will crash on every startup

Everything works near-perfectly except for the Internet - it can't connect to the developers' servers.