They will likely lock new upcoming features behind an artificial M2-required paywall. E.g. true windowed multitasking requires an M2 iPad.
They won't lock anything at all. This is probably based on the false assumption that stage manager would run fine on non M1 iPads... just because "they are fast". No, it's not a matter of CPU speed. There are 3 main reasons why only M1 can run stage manager:
1. RAM
2. storage speed (this is probably not very important, since swap is not a requirement for stage manager)
3. pro display XDR (only M1 can run it, A12Z can't). This is probably a very important reason, maybe even the most important one, although many haven't even considered it...
The fact that many could say "I don't care about the 6k Apple display" or "I would be ok with running half as many apps with Stage Manager" (both apply to me too) does not change the fact that only M1 can run stage manager with its current minimum requirements.
I don't think that Apple wanted to directly push people to upgrade from 2018 and 2020 pro iPads (although they'll happily take this "externality"). Apple makes a lot of money by selling as many devices and accessories as they can, not so much by making their devices obsolete as quick as possible. They couldn't like external display support to their own monitors of course (Studio and XDR). But they wanted external monitor support to work on THEIR displays.
For the exact same reason M1 and M2 can only run 1 external monitor, because they cannot run 2 6k displays and the internal monitor, so Apple prefers to drop support for a second monitor altogether rather than not supporting their 6k monitor when running 2 external displays.
There is basically nothing that M2 can do and M1 (not even monitor support) can't so there will be no "locked" features.