As I understand from src/security/intel/stm/Kconfig in Coreboot, STM is used for working around bugs in some features of Intel CPUs and it doesn't affect AMD.

For Intel computers, I'm not even sure if we have some models that support TXT (we could check on ark.intel.com) and if we do, I'm unsure if it depends on a working Management Engine or not.

And if for some reasons TXT works, it might be a good idea to look into it to understand if it's a security risk or if it is somewhat useful (I vaguely recall that in some configurations at least it required code signed by Intel, so it's nonfree because it's not modifiable anyway, but I've no idea if it's possible to run code without being signed, in some test mode for instance.).

Though STM also enable to run some code inside SMM for running some "vTPM", but I'm unsure if we have free software implementations of that.

Anyway for now we could leave STM as-is and care about it after the first release. This would minimize the risk of having issue blocking the release or being found after the release.

Especially if we get the release wrong people won't be able to downgrade so I think it would be better to minimize the risk of issues.