Installing Mountain Lion somewhere to run old software.

even with an iso then I don't believe you can even with a virtual machine

people found out that with m1/2/x cpus with a virtual machine only way they could run windows is if they got hold of the windows for arm cpu iso the normal intel x86 iso will not work because a virtual machine is not an emulator, so the cpu can only execute the arm binaries. and leopard, snow l and so forth never existed in arm cpu binaries

if it was me then I would by an old mac mini or the likes with an intel cpu and use that for those softwares that will not run on your m1

The iMac (21.5-inch, Mid 2011) iMac shipped with Mac OS X 10.6.6 (Snow Leopard) or 10.6.7 (Snow Leopard). If you have the Snow Leopard installation discs that came with the machine, or have a bootable backup disk that has Snow Leopard on it, you could run Snow Leopard on the 2011 iMac. (Plain Leopard is too old.) Presumably, you would want to have two startup disks, and dual-boot, rather than wiping out your High Sierra installation.

Snow Leopard is not available through Apple's support page for downloading old versions of macOS … and Apple stopped selling Snow Leopard DVDs a few years back. So if you don't still have it somewhere, you may be out of luck.

Lion and Mountain Lion will run on the 2011 iMac, and there are .DMG installers for them online. When these two OSes came out, Apple charged $20 each for them and you had to buy a key from the Apple Store. I do not know whether Apple has removed the requirement to buy a key, or not.

Stuart Mingay1 wrote:

How do I get round the "This version of OS X installer is too old to run on this version of OS X"?

Is this when you are trying to run the .pkg file when mounting the downloaded Mountain Lion "installer" or when you are trying to run the actual "Install macOS Mountain Lion" app located in the Applications folder? Theoretically you should be able to do the first even while booted into High Sierra, but you won't be able to do the second since you are already booted to a newer OS.

... also, the instructions to create a bootable installer don't include Mountain Lion. There is no createinstallmedia command in the downloaded package.

I am not aware of any instructions, although I believe I may have seen instructions at one time for Lion, but I don't believe they were simple although they probably could be applied to Mountain Lion. Most instructions you find online will most likely refer to a much older version of the installer...Apple has changed these installers over the years so it is highly unlikely older instructions will still work.

One option would be to try to boot into Internet Recovery Mode using Command + Option + Shift + R to access the online macOS 10.7 Lion installer so you can install macOS to an external drive. Then you could actually run the Mountain Lion installer while booted to the external Lion boot drive. I think Parallels may have an option to then take an image of the external boot drive so you can import it into Parallels, but I don't know if they still have a version available that is compatible with macOS 10.13. I'm not entirely certain Parallels has this option, but I seem to recall it may be a possibility. It is something you can investigate anyway, or just use Mountain Lion from a bootable external drive.

FYI, while UTM can simulate a virtual Intel computer, installing macOS that way may not be easy since macOS does have checks in place which make it hard to run in a virtualized non-native environment and it would be slow. You are much better off using your old Intel Mac for this purpose.

I believe I made a bootable USB stick from an OS X 10.6 Snow Leopard DVD, but I don't recall how I did it as it was a decade ago. I think it may be possible to use Disk Utility "Restore" feature, but I'm not certain. You can probably search online to get instructions to do so as that was probably how I did it back then anyway.

FYI, you can use an external USB DVD drive so at least you can use it with other systems. It isn't easy to open up an iMac to replace the internal SuperDrive, plus any internal drive must be made for the iMac since it has a unique mechanism due to how it is mounted internally....I don't know how compatible they are between different model years even if they use the same interface.

Edit: As for the Disk Utility "Restore" procedure, it is best to use the older macOS to do this since I was unsuccessful using it on Ventura to transfer a bootable partition to another drive recently and I started noticing issues with Disk Utility "Restore" many years ago anyway (the bootable partition was not macOS so it & the security features was not the reason for the failure).

How to download and install macOS - Apple Support