Change a Drupal 9/10 system message to say what you want it to say

The easiest way to do this is actually with string overrides.

grep your codebase for a part of the message you want to change, because you need to get the original source code to have an exact match on the string, including exactly how much it includes or doesn’t include, and any variable names it has that are substituted for something dynamic in the message.

Modules for this

Update: There are of course modules for this.

Background notes

Changes in Drupal 8.5: No more drupal_set_message

Custom system messages

In addition to changing messages you may want to wholly create your own.

I like the Configuration Construction Kit approach (that is largely what Drutopia is) but actually would prefer that alerts not be a content type, it clutters the content interface. Maybe if only

Side note on Sitewide Alerts, someone wanted to and possibly did make it fieldable: 9 - How to make a custom entity fieldable? - Drupal Answers