I released devpi-server 6.0.0 at https://pypi.org/project/devpi-server/
There are several previously deprecated things removed in this release, some changes in behaviour and several new features. Read the changelog and as always have backups ready before upgrading.
This release doesn't require an export/import cycle and it should be possible to downgrade again as long as you don't use new options on any index.
Without replicas it is a drop-in update, with replicas you have to look out for the required shared secret between primary and replicas and in case you have to down-grade for some reason make sure all files are synced. It is recommended to upgrade either everything at once, or the replicas first and the primary last.
Regards, Florian Schulze