I have a 2008 mbp that can unofficially run the latest macOS. Will the move to AS make this impossible?
Usually Apple supports a given Mac with OS updates for at LEAST 5 years. Quite often longer.
So my guess is 5 years from whenever Apple sells the last Intel Mac they may stop including Intel support in new versions of macOS. At that point unofficial updates will no longer be possible.
As long as Intel is supported by Apple; it is just an issue of working around any GPU and hardware limitations to "hack" older Macs into running newer versions of the macOS. So Macs with certain GPUs tend to have an easier time with this hack. DosDude and similar hacks won't be going away anytime soon.
This is just a guess. But it's based on 32 years of experience with Apple
Will macs running AS be prohibited from ever being able to run unofficial os's as we currently are able to on intel machines
This is a VERY different question. It's hard to say how much more "locked down" Macs using M1 and future chips in the series will be in comparison to Intel Macs. I would guess that they will be MORE locked down for sure. But Apple has been doing a reasonable job of balancing the more open source and "free as in speech" origins of macOS (BSD/Mach/UNIX, etc.) with their desire for more security and walled ecosystem. The Mac has remained pretty hackable in comparison to what they COULD have done. Because of this, it is at least possible future Macs will still be somewhat hackable.
You have to give them SOME credit for Apple basically ignoring the Hackintosh community, for example. Most companies in Apple's position would have worked tirelessly to block them.
It seems like this is the primary division between their iOS devices and macOS devices. iOS fully locks down everything. But there are quite a few pretty hackable things you can do on the new M1 machines that Apple could have very easily blocked. I think at least some engineers inside Apple see the advantage of a hacking community.
I'm willing to give Apple the benefit of the doubt on this; although I'm sure many open source/free software advocates will not.
tl;dr To answer your question; I'd give this a 50/50 chance.