If you are editing a file, add a comment to the bit you edited stating you did it and when (and if there’s an internal story/ticket reference)
If you are performing maintenance on a system add a note to the motd so people see it when they login. Be specific about what they should expect to see and what, if anything, should NOT be out of the ordinary (services that should still be running).
If you deprecated an old script, make the script dump an error telling the person what happened. Make the script fail to run if it shouldn’t – don’t just expect that people will not run it, they will.
Think about how someone might come in contact with your change and be defensive, they’ll appreciate it.