I'll copy the answer I provided in the main thread over on !meta@programming.dev, but to also add, the tool does not monitor all users. It only monitors newly created user accounts on programming.dev for x amount of time so that we know that the account isn't being made to abuse or spam. We do not want to have this tool monitoring regular users.
Copied reply about why DM monitoring is needed:
From the PR standpoint, I don't think it's worth it, and it'd be better to just leave it on reports.
The problem is that we can't rely on reports of DMs since lemmy doesn't federate them to us.
E.g.
-> troll@programming.dev makes a new account and sends harassment to victim@lemmy.instance
-> victim@lemmy.instance reports the DMs from troll@programming.dev
-> We, the admins of programming.dev, do not see this report because lemmy does not federate the DM report and troll@programming.dev can continue harassing others because we never find out about it.This isn't just a theoretical, it happened just last month that one of our users (1 day old account) sent rape and death threats (which were reported), and we found out about it by pure chance when talking to admins from the other instance.
And just to clarify, the tool only automatically monitors new accounts, i.e. accounts that are being registered today. If you account is more than a few weeks old, the tool doesn't monitor any of your activity.

It's an open issue on github from 2024, it doesn't seem to be a priority. This tool allows us to react faster than reports though, and hopefully remove some problematic content before it's seen by others.