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Excalidraw is free & open source software though, which IMO makes it deserve more attention

Oof, source available, that is an oversight on my end. Or was there a recent license change? Off the shortlist then, unfortunately.

Yep, license change from Apache 2.0 in December 2023.

Have any forks from the FOSS version picked up any traction?

What do you base this on? The main repository (https://github.com/excalidraw/excalidraw) states it's MIT and this has been unchanged for 5 years.

EDIT: I guess you were talking about tldraw rather than excalidraw.

tldraw's source is available fwiw