Hide File Extensions not maintained since Sequoia 15.4 upgrade on M1 Mac mini and M3 MacBook Air

The answer to the question is out there on the interwebs, but I'll share what worked for me. You need to be comfortable with the command-line, using the Terminal app (I recommend iTerm2, but I digress). There is a command called defaults that lets you make changes to the system defaults, without having to dig through app settings. It does come with its own learning curve, and you can always read about with man defaults in the terminal if you want to learn more about it.

The specific setting is the global attribute AppleShowAllExtensions, which should default to false. Unfortunately it seems to be getting stuck to true even if using the Finder's settings to change it. Since the attribute defaults to false, we can just "delete" it, which the Finder sees as "use the default." The command specifies the global context with the option "-g". Once the change is made you need to restart the Finder, which is what the killall command does; you should no longer see the extensions. Don't worry, the Finder automatically restarts.

defaults delete -g AppleShowAllExtensions && killall Finder

This has been bugging me for a while, finally realized that default was the answer, which I confirmed by a quick DuckDuck search to confirm I had the correct setting.