Users of my (free, open-source) app seem surprised to learn that we've got zero insight into usage patterns. There are situations where a small amount of anonymous telemetry would be extremely helpful but I'm not going to touch it with a barge-pole. Opt-in makes the data useless - not just in terms of the huge drop in quantity but because of the fact it introduces a huge bias in the data selected - the people that wo…
This can only ever be opt-in if you want to stay on the legal side of the GDPR (and equivalents in other jurisdictions). You can ask, but the default needs to be "no" if no answer is given. I provide telemetry data to KDE, because they default to collecting none, and KDE is an open-source and transparent project that I'd like to help if I can. If I used your app, I would be likely to click yes, since it's open-source…
The GDPR only applies to personal data. You can collect things like performance data without opt-in (or even an opt-out option) as long as you are careful to not collect any data that can be used to identify an individual, so no unique device IDs or anything like that. Of course, you should be transparent about what you collect. You also have to be careful about combinations of data points that may be innocuous on their own but can be used to identify a person when combined with other data points.