See also T258098 and T252812
=== Problem
The DBAs have expressed concerns about the ever-growing size of the `watchlist` table. In #community-tech's research for #expiring-watchlist-items, we discovered the accounts with the most watched items are almost always bots, and appear to be only because the "add pages and files I create to my watchlist" preference is on by default. It's not surprising that bots create so many pages, since naturally they operate at a higher rate than humans.
=== Proposed solution
Automatically turn off auto-watch preferences when an account is added to a user group with the `bot` permission. This seems rather uncontroversial, as bots conceivably have little use for the watchlist. If they do need it for whatever reason, the operator could simply re-enable the preference.
An Echo notification could be sent when this occurs. This is important because some communities temporarily grant bot rights to humans for them to perform some large-scale task, and these humans should be aware of changes made to their preferences.
**Some quick data**
For perspective:
* commonswiki: 14% (~22 million rows) of the watchlist table are owed to bots, which appear to be bots that upload files and simply have the default preference set to watch them
* wikidatawiki: 1.1% (~91 million rows) -- probably bots that automatically create items after articles on Wikipedia are created
* enwiki: 4.7% (~10.3 million rows) -- e.g. counter-vandalism bots that create User talk pages when issuing warnings
* mgwiktionary: 99.8% (~13.2 million rows) -- a single bot mass-created nearly every entry on the wiki