The chrome://flags/#side-panel flag does not exist anymore.
The question is asked here:
The chrome://flags/#search-web-in-side-panel flag does not solve the problem:
You can't anymore.
They keep adding useless buttons without the possibility of hiding them.
They don't care.
Just switch browsers.
Try something new Edge and Brave are good Chrome alternatives and Firefox is also great if you don't mind not being on Chromium.
You'll probably find edge the best though. Its essentially chrome but way better nowadays.