Anyone using Unity on macOS who got it working with .NET4 (or higher)?
You can see this if you go to Edit → Project Settings → Player → Configuration, and look if you have more options than in my attached picture (ie a .NET4 option)
API compatibility level is not the same as runtime version. All modern Unity releases use mono/IL2CPP runtime equivalent to .NET 4.x, whereas in the past there was a .NET 3.5 runtime that was already deprecated by the time of 2018.3. The API compatibility level just sets what API surface you can use in the project. The “.NET Framework” option takes .NET Framework 4.8 and adds on .NET Standard 2.1 capabilities, with some .NET Framework functionality being Windows-specific.
I’m guessing you’re just trying to upgrade Input System and don’t see the newer releases as updates in the Package Manager. This happens because Unity packages have a validation process that doesn’t show newer package releases if they haven’t been checked for any issues with that editor version. You can always manually install a package by name and version. There’s nothing to be concerned about with respect to runtime version being .NET 4 in all currently supported versions, all of them use that.