Something like that. There need to be two headers set to enable this threaded build:
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
Itch doesn't set those. Chrome so far only showed a warning, but with the recent v91, it is mandatory like it was in firefox for a long time.
So, i'm very thankful for your comment, otherwise it would have remained unplayable for many people.