We are sometimes using new \MediawikiCoreClass() instead of adding a use clause. There should be no absolute class references with full namespace in our code. Add a PHPCodeSniffer rule to check for this and fix the remaining cases.
Is this about use keywords or @uses tags in tests? The first doesn't need discussion, in my opinion. The second does.
I'm preferring new \TestUser() (to give an example) over use TestUser; in very, very few cases. For a single reason: PHPStorm removes some use lines when pressing Ctrl+Alt+O. But I don't mind (any more) if we are changing all of these to use use.
In general there should not be any \ in the code. I do have the impression that we all agree on that. Therefor I don't think this is an issue that needs to be tracked in Phabricator. I suggest to close this.
@Lydia_Pintscher I noticed your question, two years late, but oh well :)
In general I think we are doing fine CS wise. I agree with Thiemo that there are some cases where using the FQN is fine or even preferred. Given that, I'm not sure a sniff will be more helpful than annoying. However I'm not against trying it out and seeing what happens.