There is an Ubuntu ARMv8-A release (Commonly called AArch64) and there are many Linux-based distros running on AArch64 hardware. Probably wont use any extras on the Apple M1 SoC but it would run nonetheless.
I guess we'll see when my M1 air arrives this month. It's my understanding that by default they won't boot anything not signed by Apple, but I'm hearing scattered reports that there may be some command line configuration tool options that can cause it to disable boot security and boot unsigned stuff. It's certainly the default configuration, and on all of their other arm64 platforms they've shipped so far (iPhone/iPad/ATV/Watch), disabling the boot security is not a configurable option.
