Edit > Cut Frame works on a locked layer while Edit > Paste Frame and Frame > Clear do not.
This seems like an inconsistency. I would expect to not be able to remove all the pixels from a locked layer.
When a layer is locked I expect it to be protected against any sort of pixel content editing.
Ability to change a locked layer’s blendmode, opacity percentage, and effect settings is surely fine, though - the actual content of the layer isn’t being affected.
A little more grey, but you can also merge down a locked layer, as well as merge another layer down into a locked layer. This also seems fine, I guess. If you’re merging layers you really need to pay attention to what you’re doing anyway. If I was to merge a layer I had previously locked and intended to NOT change I would probably blame myself entirely for simply not paying enough attention.
There is one undesirable behavior when merging down a locked layer, though - when you Undo the merge, the layer’s lock doesn’t come back; PMNG forgets it was locked.