@codinghorror recently noticed that old inactive users were not getting purged. Turns out this was because the inactive users had a welcome message from discobot which prevented them from getting purged.
We will now purge inactive users even if they have a message from non-human users (system, discobot) as per:
https://github.com/discourse/discourse/commit/7550e9ff958d13465d0382f6daab193ff5eda7fb