I think we should show this notification only to users who visited their Homepage and saw the mentorship module there (which excludes the cases of en.wp and es.wp). Otherwise, we shouldn't notify them.

In theory, claimed users are claimed by someone who told them that they will claim them. It is part of a conversation that certainly involves the mention (and link) to the homepage. Let's have mentors tell their mentees that it is done.

Thank you, @Urbanecm_WMF for thinking through all of the implications of T327206!

In my opinion, the message is generic enough that it doesn't seem odd to show to newcomers who previously didn't have access to the mentorship module. It's perhaps not perfectly tailored to that use-case, but it still makes enough sense to not be misleading.

"(username) is you new mentor"

To me that could mean I have a new mentor or it could just be a message informing me that I now have a mentor.

But I'm fine with whatever @Trizek-WMF and you think is best here. I just want to make sure we aren't spending too much time on addressing what should hopefully eventually be an edge case (once mentorship increases on English and Spanish Wikipedias).

Change 937897 had a related patch set uploaded (by Urbanecm; author: Urbanecm):

[mediawiki/extensions/GrowthExperiments@master] ChangeMentor: Refactor the notification conditions

https://gerrit.wikimedia.org/r/937897