On the computers (i.e. mac mini, macbook, iMac) Apple ships an unlocked bootloader and we can just install our own bare metal code very early on using their official tools.

On the iPad Pro that bootloader is locked down and running Linux on there is unfortunately very unlikely to ever be possible.

It would be possible, though very difficult, to turn the OS into a staging bootloader, and load another OS bare-metal from there. Still insanely difficult though.

Usually, once you can run code in kernel mode it’s just a matter of bringing the hardware back to a sane state.

Unfortunately, “just” running code in kernel mode is incredibly hard on the iPad. There’s e.g. a hardware mitigation that turns a memory region to read-only and also only allows kernel code to run from this region. This mitigation is locked down and cannot be disabled once XNU is running on the iPad (http://siguza.github.io/KTRR/).

On top of that page tables are also protected by a “kernel within the kernel” (https://blog.svenpeter.dev/posts/m1_sprr_gxf)

Getting past all that is going to be very hard if not impossible.

I thought the first-generation iPad pro was checkra1n-compatible? Isn't checkm8 a bootloader exploit?

Yes the first gen iPad Pro can, and the second generation one too. The target user base for Linux on those might be too small though.

But all that's needed is a bootloader exploit...

Chkm8 was the first of its kind since the early days of iOS. It's not impossible but we'd have to get very lucky.

which is not impossible but very unlikely.

It’s fun to do an’ all, but the lack of drivers really made the experience half-baked for me when I realized I couldn’t use any of the badger’s limbs.