Has anyone said definitively that it wouldn't be open-source in the future? I know sometimes this is due to things like joint or contract development where the lawyers need to hash out the rights first.
Curiosity leading me to try to answer my own question with an educated guess: Pylance does appear to use the ONNX runtime, so it seems probable that it could be using ML models for type information to feed to Pyright developed by or in concert with IntelliCode efforts and that research effort of IntelliCode does fit what Microsoft is currently keeping proprietary.
I'm curious what research/IP has lead to this being a source turducken of open-closed-open, but it's not entirely closed.