you opt in by using the product. especially if its open source.
As far as I'm aware, the contract of OSS is "you can fork this to change it, if you don't like it" (license permitting).
A big part of the reason people advocate for OSS is so they can read the source code to understand what's going on, and change the parts they don't like. Telemtry sits squarely under that usecase.
I agree that it's good manners for projects made users aware of telemetry, but there's no "contract" that implies they have to.