There's also the nvidia enclave stuff. If you trust their hardware, then you can also trust its cryptographic proof that the data was not seen by the inference provider.

Ignoring whether or not there could be backdoors, security holes in the Trusted Execution Environment stuff are still being found on a regular cadence.

I'd guess it's good enough to avoid bulk surveillance (and therefore extraction of training data, which I guess is the attack that Notion was running when they screwed up and leaked the data), but that's about it.

Apple's stuff is a similar story. (And so are VM enclaves, which have been around longer.)