Sharing answers to top q's we’re seeing about the thumbnail experiment: – Custom thumbs aren't going away – 99.7% still see the creator's custom thumbnail – A lot of creators use auto-generated thumbnails, this test helps improve them for those using them: https://goo.gl/A7oDmr
You should have a generic opt-in for future experiments like this. You can encourage creators to do this by granting them early access to certain experimental features in exchange (such as the Teespring shelf closed beta earlier this year). Consent is inconvenient, but essential.