However, if you have database access, you can avoid going into every forum admin area by doing it with a query:
DELETE QUICK FROM VISITOR_LOG WHERE LAST_LOGON < CAST('2010-01-01 00:00:00' AS DATETIME)
To first check how many rows would be removed from each forum before running that, you could use:
SELECT FORUM , COUNT(*) FROM VISITOR_LOG WHERE LAST_LOGON < CAST('2010-01-01 00:00:00' AS DATETIME) GROUP BY FORUM
You might be able to change the condition to a dynamic value like so:
SELECT FORUM , COUNT(*) FROM FROM VISITOR_LOG WHERE LAST_LOGON < CURRENT_DATE() - INTERVAL 9 MONTH GROUP BY FORUM
If that works, you could put the line in a .sql file, then schedule a command that runs it against the database using the server OS's scheduling facilities (i.e. cron/systemd/whatever).
There will be no release for this (or anything else now) and you upgrade at your own risk.
I like the sound of that. Going forward, it will be my policy.