There is the Debian popularity contest: popcon.debian.org , which tracks how many Debian users have a given package installed.

popcon is intirely opt-in though, quite different from automatic telemetry that the user doesn't even knwo about

Not really, there is a lot of projects doing this as an anonymous statistic, for example InfluxDB sends anonymous statistics about your InfluxDB instance to their server. This way they have overview/statistic of usage (they are visualize it on word map, looks amazing), together with data like OS, version you use etc... You can disable this in config https://influxdbcom.readthedocs.io/en/latest/content/docs/v0...

Frankly, there could be some value to OSS contributors in saying 'There is Telemetry' very up front, but also in the same breath telling people that instructions for disabling Telemetry is in the Docs.

Could help lower the number of GH issues that are filed because people don't read said docs...