• Why isn't there any HiDPI resolution on my M1 Mac ?
    At first, just remember that SwitchResX never was able to enable HiDPI. HiDPI can be enabled and SwitchResX will show these resolutions, but SwitchResX itself isn't designed to enable the HiDPI capability on ay Mac.

    On Intel Macs, any monitor could show HiDPI resolution, sometimes directly, sometimes after a command line was entered to enable HiDPI mode.

    On M1 Macs, not only the command line doesn't work any more, but HiDPI is explicitly reserved to specific monitors.
    Apple intentionally disables all HiDPI capabilities for all monitors that aren't 4K native, or don't have a pixel density of at least 160dpi (on the horizontal size as reported by the monitor).

    It is intentional, as it worked on the DTK (the Mac that was used as prototype for M1 Macs) up to BigSur Beta version (ß6) in September 2020. Beta 7 removed this possibility from Monitors that aren't 4K, and then beta 9 removed it from monitors that aren't 4K Native.
    Reporting this as a bug or as feedback to Apple resulted in them answering it's acting as intended (see radar FB8715490 for those who can access this). This proves it's intentional, not a hardware limitation, and certainly not a bug that you should wait to be solved.

    From the best of my knowledge, HiDPI resolutions are enabled on M1:
    • on internal MacBook screens, because the pixel density is higher than 160
    • on 4K monitors showing a 4K as the native resolution. Note that from Apple words: "the mere presence of a 4k timing does not mean it’s a 4k display and because of this we don’t expose HiDPI modes.". The basically means the monitor must show 4K as its native resolution. Some 4K Dongle are 4K capable, but not 4K native
    • on any other monitor, whenever it mirrors an other monitor where HiDPI is available (example: a 2K screen mirroring the internal M1 MacBook screen). This again proves the limitation is intentional, since the video driver is then completely capable of handing HiDPI resolutions. This has given some clever developers to develop software solutions to add virtual 4K monitors to mirror your screen onto, like BetterDummy.
    • (last bug I noticed: if the monitor reports that it also supports 1280x1024 @ 60Hz, as standard or established timing in the EDID, even if it is a 4K native monitor, then HiDPI will not enable. This is likely to be a completely dumb bug in macOS, as I don't see any reason for this behaviour - FB9869969)