• Why isn't there any HiDPI resolution on my Apple Silicon Mac ?
    This covers Apple Silicon Macs only

    Beginning with macOS 12.4, the behaviour changed on Apple Silicon Macs.
    It seems to now be in par on 4K monitors or in non-4K monitors. HiDPI resolutions get again enabled, up to the native resolution (ie. If your Mac uses a 2560x1440 monitor, you'll be able to get a 2560x1440 HiDPI resolution (by creating a 5120x2880 resolution), or anything lower. You cannot get a 2840x2160 HiDPI resolution in this case (which would be useless anyway).
    This matches the behaviour of the 4K displays now.

    For history: before macOS 12.4 Monterey, HiDPI was explicitly reserved to specific monitors.
    Apple intentionally disabled 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 was intentional, as it worked on the DTK (the Mac that was used as prototype for Apple Silicon 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 proved to be 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 were previously only enabled on Apple Silicon:
    • 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 Apple Silicon 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
    • (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)