There's some combination of circumstances which prevents it. I think specifically it's encrypted swap, a kernel setting and maybe something else... I can't remember, except that when I looked at it I couldn't be fucked trying to get it sorted.

Yeah... I don't exactly the relationship between S0ix, s2idle, and "modern sleep" - I'm sure there's some overlap but to me it's all the same: it's not old-fashioned S3 sleep, and it just doesn't work properly. I remember reading that with my laptop specifically, S3 was replaced with S0ix so I'm pretty sure that's the right term in this case ;)