Sideloading on the M1 is fairly easy. @marcan42, who is developing Linux for M1, is very clear that Apple actively went out of their way to make sideloading possible in a secure manner.

Also, for everyone who says Apple is going to lock MacOS down, that's not going to happen with the M1 at least. The ability to run older versions of MacOS, alongside Linux support coming, means it's not happening. Despite what the conspiracy theorists say.

Linux on M1 and sideloading OSX apps are fairly orthogonal. Bootloader/sideloading term mixup maybe?

They are not orthogonal. If you can install Linux, then you can sideload apps on macOS. It might involve invasive XNU patching with a hex editor, but it can be done.

It's not that involved, by the way. There's a command that you run to turn off gatekeeper [1].

[1]: https://disable-gatekeeper.github.io/

Exactly, and this goes beyond mere self-signed code (Gatekeeper) and into things like SIP and private entitlements. As long as the user controls the bootloader, the user can also patch the OS to do whatever they want.

I think the OP is pointing out that installing Mac apps from places other than the App Store is not what most (if any) Mac users would call "sideloading". We just call it "installing." :)

It's 2 separate points, Apple explicitly said they allow sideloading (I wouldn't even call it side loading, it's just "install from outside the app store" and "allow unsigned applications if you want") and booting any other OS is allowed, there is even some amount of Apple documentation on how to do it, and success with both BSD and linux. Drivers are of course an issue.