All known callers provide a user, except:

1. FileRepo::getUploadStash accepts an optional user, defaulting to null, and passes that on
2. UploadFromStash::__construct accepts an optional user, defaulting to false, and passes that on
3. The cleanupUploadStash maintenance script doesn't pass a user
4. UploadStashTest tests don't pass a user

See codesearch: https://codesearch.wmflabs.org/search/?q=new%20UploadStash%5C(&i=nope&files=&repos=

For #1
All known callers of FileRepo::getUploadStash already provide a user: https://codesearch.wmflabs.org/search/?q=getUploadStash%5C(&i=nope&files=&repos=

For #2
All known callers for UploadFromStash::__construct already provide a userhttps://codesearch.wmflabs.org/search/?q=UploadFromStash&i=nope&files=&repos=

For #3 and 4: Test will hide deprecation, script to be updated

Accordingly, not passing a user to the constructors of UploadStash or UploadFromStash, or to FileRepo::getUploadStash, should be hard deprecated