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The AM hierarchy (constant-round interactive proofs) collapses to AM (Babai-Moran '88), but we don't yet know whether NP=MA=AM.


Perhaps this recent result (Jan 2020) showing that MIP* = RE.


Ineresting result from Quantum Computing, though, If it fits into your requirements of what hierarchies you are looking at, is at discretion. The QMA hierarchy collapse result of Harrow, Montanaro where QMA(2) =QMA(k) for k >= 2.


The analogue of the $\mathsf{NC}$ hierarchy for algebraic circuits is known to collapse to the second level. That is, algebraic circuits of size $n^{O(1)}$ computing a polynomial of degree $n^{O(1)}$ can be rebalanced to have depth $O(\log^2 n)$ while only increasing the size by a polynomial factor. This is due to Valiant, Skyum, Berkowitz, and Rackoff. It ...

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