So really we should be finding a way to un-disable these accounts: cf T123139 / T100400

Generally speaking though, you're right.

I assume there are internal WMF procedures to un-/reassign tasks as part of offboarding & Co.

It's not a standard part of offboarding (according to the internal pdf I just looked at), thus it's up to the responsible people manager or product manager.

Contributions (via an SQL query) to https://phabricator.wikimedia.org/diffusion/OPUP/browse/production/modules/phabricator/templates/project_changes.sh.erb welcome.

In the long run maybe https://phabricator.wikimedia.org/maniphest/report/user/ could share some options like the "Disabled" dropdown that the Advanced people search offers (if we got "report" to work reliably at some point). But that would be a request to upstream.

In the short run, we could add the output of

SELECT u.userName, count(u.userName) AS n FROM phabricator_maniphest.maniphest_task t JOIN phabricator_user.user u WHERE u.isDisabled = 1 AND t.ownerPHID = u.phid AND (t.status = "open" OR t.status = "stalled") GROUP BY u.userName ORDER BY n DESC;

to add to the weekly project_changes.sh.erb email and act upon (as I do read that stuff).

Change 351011 had a related patch set uploaded (by Aklapper; owner: Aklapper):
[operations/puppet@production] List disabled user accounts with associated open tasks in weekly Phab email

https://gerrit.wikimedia.org/r/351011

Change 351011 merged by Dzahn:
[operations/puppet@production] List disabled user accounts with associated open tasks in weekly Phab email

https://gerrit.wikimedia.org/r/351011

Note to my clueless former self: When the weekly script lists tasks but the web UI does not list any tasks, tasks in Spaces which I cannot access are the reason.