The same thing that has previously happened with discontinued Intel Macs or PC hardware that is no longer supported by Windows.

You can still boot Linux natively on it.

i have no idea how you think apple silicon will discontinue support for itself.

the hypervisor framework documentation, that i linked above, is pretty clear; the linux builds for arm run extremely fast on apple silicon because hypervisor framework gives them hardware access, in some subsystems directly (memory mapping for example), in other systems through a shim (graphics hardware).

Since the hardware will no longer get security updates for the firmware you should stop using it and move to a newer MacBook.

Linux doesn't solve the issue of not getting security updates anymore. For example the FCC does not allow letting users write arbitrary firmware they can run on their devices so this means you legally can't update the Bluetooth or Wifi firmware and need an update from Apple.