Hello @Iniquity, thanks for the question!
Fortunately, it is possible to create this list quite easily, because the Growth team decided in late 2021 to publish all mentorship data via dumps.wikimedia.org. Each Saturday, mentorship data are released at https://dumps.wikimedia.org/other/growthmentorship/. It is then possible for anyone interested to download the data and to build on top of them.
It is also possible to query the mentorship data via the API. For example, if I want to see all mentees assigned to you at Russian Wikipedia, I can do this: https://ru.wikipedia.org/w/api.php?action=query&format=xml&list=growthmentormentee&gemmmentor=Iniquity.
To create a list of users with at least one mentee, regardless of whether they are currently signed as mentors, the dumps are more convenient, as they provide access to the complete data.
To make it easier for you, I created an example PAWS notebook which generates this list.
You can find the list at the end of the notebook's page, together with the code I used to generate the list.
Let me know if the list is useful to you, or if you have any questions about how to make use of the mentorship data -- I'll be happy to help!
Best wishes, Martin Urbanec