one of Niantic’s official Support Accounts, NianticGeorge, has explained the delay behind the new update, version number 0.71:
We are aware of a latency issue affecting battling in Pokémon GO version 0.71.0. We are currently investigating potential resolutions. Although this issue only affects a small subset of users, we are suspending the rollout of the release while we continue to work towards delivering a better experience. – source
In the comments of the source reddit thread, George also explained how the update rollout process works behind the scene:
When we release an update, we release it to a small percent of the userbase at first. We start at 1% and work our way up to 100%. We monitor our reporting metrics, user-submitted bug reports, and sites like reddit for signs of issues during the rollout process. If something comes up (like it did in this release), we’re able to pull the plug on the update before the entire userbase is affected.
It’s not yet clear when the propagation of the new update will continue, but we’ll let you know once it does. In the meantime, APK Mirror has the 0.71 APK if you wish to experiment. We’re using it in the day to day gameplay and we must admit, it is laggy.
We expect that the lag issues will be resolved in the next 10 days, prior to August 31st. This update is required for the Super Incubator release, indicating that any encountered issues will be dealt with promptly.