On Special:PrefixIndex, e.g. https://en.wikipedia.org/wiki/Special:PrefixIndex/User:Od1n/
Previously, the input "Display pages with prefix:" contained the current prefix (e.g. "Od1n/", and namespace "User"), which let quickly editing the value.
This input is no longer filled :'(
Another case in point, to know what is the current prefix, there is no more information on the page, you have to look at the page title...