Welcome to Ubuntu MATE community!
The Collection is a new feature of MATE 1.26 which is shipped in Ubuntu MATE 21.10.
Requests to make it configurable were already filed as https://github.com/mate-desktop/mate-menus/issues/101 and Bug #1941942 “New Collection menu should be configurable” : Bugs : mate-menus package : Ubuntu .
Temporarily way to remove Collection menu category is to run the following long command (copy and paste whole code to the terminal in one shot):
mkdir -p /home/$USER/.local/share/desktop-directories/
cat <<EOF > /home/$USER/.local/share/desktop-directories/mate-collection.directory
[Desktop Entry]
Name=Collection
Comment=The most frequently used shortcuts
Icon=starred
Type=Directory
NoDisplay=true
EOF
and then reboot.