Ubuntu MATE 20.04 LTS is End of Life - Ubuntu MATE 20.04 LTS reaches end of life
Ubuntu 20.04 LTS was released in 2020-April; and ends its 5 years of standard support in a number of weeks; so your release should have you considering a release-upgrade path, unless you're close to the end of your project anyway.
Also note that GTK4 is where the development is, with GTK3 in maintenance mode only; being there for legacy apps that haven't been ported to the newer version yet.