Hi Eric,

Ok, Think I found the differences:

Stockfish 16:

Yes, it is possible to play Stockfish 16 without the NNUE (Efficiently Updatable Neural Network) evaluation file, but it is not recommended as it will play significantly weaker or perform poorly. As of version 16, Stockfish completely removed the old "handcrafted evaluation" (HCE), making NNUE the sole, mandatory evaluation method.
Here are the details on running it without the network:
Performance: If the .nnue file is missing or disabled, Stockfish 16 may not have a valid evaluation function, causing it to act erratically or play very poorly.

Stockfish 15.1:

Performance Differences: While Stockfish 15.1 is designed with NNUE for maximum strength, it still retains the classical evaluation method. However, playing without NNUE makes the engine noticeably weaker in positional understanding, as the NNUE is responsible for a significant portion of its Elo gain.
"Hybrid" Option: Some versions of Stockfish 15.1 allow you to use a "hybrid" mode, which combines the classical evaluation with the NNUE network.

This could be why 15.1 is quoted as 16 without NNUE was no longer stable? And with 16.1 it was fully removed.

Anyway in summary if you play without NNUE its best to just play Stockfish 11:

Historical Context: The last pure, non-NNUE versions were similar in strength to Stockfish 11, which was already very strong, but cannot compete with modern NNUE-enabled Stockfish.

Regards