Audacity 2.3.3 is the current version of Audacity. You can get it via the Audacity website: https://www.audacityteam.org/download/windows/
“rms-normailize.ny” is a “Nyquist plug-in”. That is, a plug-in for Audacity, written in the “Nyquist” programming language.
The latest version of this plug-in may be downloaded from this page: https://forum.audacityteam.org/t/rms-normalize/45334/1
You should definitely use the latest version of this plug-in - I rarely update plug-ins without good reason (such as to fix bugs).
Audacity 2.3.3 has a tool that makes installing Nyquist plug-ins very easy - If I recall correctly, Audacity 2.3.1 does not have this.
https://manual.audacityteam.org/man/nyquist_plug_in_installer.html