I've been using Wayland for quite a long time already. Since one of my displays provides invalid EDID data, I'm unable to set the resolution properly, because the OS think that the device supports only 4 resolutions: 640x480, 848x480, 800x600 and 1024x768.
Until lately, I used a hacky solution – forcing the kernel to load a specific EDID for specific output on boot. That way, I was able to use only resolution specified in that EDID.
But lately, that solution stopped working on my machine, and, since Wayland does not support adding custom resolution, I was forced back to X, because thanks to xrandr I'm capable of doing so.
I think that Wayland should have an option to replicate that feature.
adding custom video modes is a feature of your particular Wayland display server.
All configuration is up to each Wayland display server individually. They could agree on a mutual configuration standard, but that would be out of scope for Wayland as understood by the bugzilla product/component in this bug.
Loading an EDID file through the kernel seems like a good solution to me, so it might be worth figuring out why it stopped working.
If you want to make a configuration with your display server, please consult your specific display server documentation.
FWIW, for years people have come up once in a while suggesting that Wayland should replicate X11 RandR, but it has never gained enough traction with the major desktop projects to become a serious proposal. Sorry.
I think two major counter-arguments are: a) access control, we do not want arbitrary applications changing video modes at will, and b) once the Wayland display server accepts connections so that an app could program a video mode, the server has already initialized the display hardware so it's too late and will lead to glitchy user experience. Drafting such an interface would also be a considerable amount of work for a feature that most big desktop environments already implement by some other means.