The current version is Audacity 3.1.2. You can get it here: Audacity ® | Downloads
The 3.1.2 version has a lot of fixes since 3.0.3. There won’t be any changes to 3.0.3 from the Audacity team - the version number is bumped after changes.
The Noise Reduction code has not changed significantly in many years, though all effects seem to be a bit slower in 3.1.2 AppImage compared to 3.0.2 installed from the Ubuntu repository.
I’m actually running Zorin 16, which is a skin on top of Ubuntu 20.04. I believe it uses the gnome windows manager and desktop,
but it tweaks quite a lot of the desktop layout settings. Anyway, there is no x button at the top right in this particular modal popup, although there
is generally in other windows, such as the Audacity main window.