The Fresh Produce section is where these are usually announced—although sometimes also the hidden category, Operations, the email newsletter, or the blog. (Unfortunately the much-appreciated Infrastructure Log seems to have been retired now, but that was a fifth source of early hints in the past.)
I think @khuezy is referring to the following in particular from Fresh Produce:
https://community.fly.io/t/faster-more-reliable-image-deploys-for-all-remote-builders/26072
Although changing the behavior of tags wasn’t intended, it appears…