WWDC included discussion of "Reduced security" mode, which allows downgrading to old signed versions of MacOS, but I don't remember any disclosure of plans to permit arbitrary operating systems at that point - do you have a pointer?

Edit: The transcript at https://developer.apple.com/videos/play/wwdc2020/10686/ discusses reduced security, but doesn't say anything about booting arbitrary code.

You found the right talk, but it’s actually only waved at and not explicitly discussed so it’s not in the transcript. At about 19 minutes the engineer mentions that a csrutil will be able to do more things and on the slide disabling secure boot is one of them. (This actually got moved into bputil, but the intent was there at least.)

That indicates that csrutil would be able to do /something/ with secure boot policy, but I really don't interpret that as a commitment to being able to boot arbitrary operating systems - if we were still unable to do so today, I don't think you'd be able to put up a strong argument that Apple had broken a commitment.

The only thing I can plausibly interpret that as is “we’ll let you turn off secure boot” (I mean, what else can you do?). I agree that Apple never committed to anything beyond that (and I am pleasantly surprised as to how much effort and documentation went into what they have currently given) but I think it is a reasonable interpretation from the slides that getting some sort of OS loaded that isn’t Apple’s is something they had advertised.