Multi-window mode no more default.
The Main Windows page (https://docs.gimp.org/2.10/en/gimp-concepts-main-windows.html) says:
When you open GIMP for the first time, it opens in multi-window mode by default. You can enable single-window mode through Windows → >Single-Window Mode) in the image menu bar. After quitting GIMP with this option enabled, GIMP will start in single-window mode next time.
AFAIK, multi-window mode no more default, it looks like we need to update the page:
Even, the Image Window page (https://docs.gimp.org/2.10/en/gimp-image-window.html) says quite similar to the Main windows page:
GIMP user interface is now available in two modes: multi-window mode (default), and single-window mode (optional, through Windows → Single-Window Mode. But, if you quit GIMP with this option enabled, GIMP will open in single mode next time).