I've been using userData to store some asset state info. UserData was chosen instead of a hidden parameter because userData can be set on a locked node, and should be saved to hip file or HDA just as a parameter.

And it all works... except it turned out that userData on locked nodes is not saved to the hip file. So an asset that consists of assets that store part of the state in userData will not store it's state to hip file, and on each scene load the userData on internal nodes of the asset instance will be reset to what it was during asset creation.

My only question is - is this the intended behavior for userData?

Honestly, I don't think this is intentional.
I'm using version 20.0.896, and from what I’ve seen, userData is saved together with the scene.
The data that isn't persistent between sessions is cachedUserData, as mentioned in the documentation here:
https://www.sidefx.com/docs/houdini/hom/nodeuserdata.html [www.sidefx.com]