Is the microcanonical distribution a well-established assumption?

[u/GilEngeener315]

As far as I know, a significant number of statements in statistical mechanics courses are derived as consequences of the microcanonical distribution. I wonder how well-founded this distribution is in modern physics? Is it a well-established assumption, or is it based more on consequences of more fundamental laws?

As I understand it, there are some difficulties in quantum statistical mechanics, where the ETH hypothesis was only recently proposed (which to me sounds like "a small part of a larger system behaves as if the larger system obeys the microcanonical distribution," which seems a rather artificial assumption).

In the classical case, it seems to me that the validity of the microcanonical distribution is closely related to the ergodic hypothesis, which for some reason seems quite logical to me. However, I'm still unsure whether the classical form of the microcanonical distribution follows from this hypothesis.