As you know Ubuntu Mate 19.10 come with a kick-ass feature, no more screen tearing on the nouveau driver. Along with no screen tearing it also comes with faster desktop response.
But as observed on one of my first upgraded systems it did not work, as it already contained an Nvidia proprietary driver. So here is the guide on how to remove any Nvidia proprietary drivers and return to nouveau. This guide was tried and tested on two systems, one already on 19.10 and the other before an upgrade from 19.04.
I wrote this guide to make it easy for others as I invested quite a bit of time finding a correct solution to this situation.
Search for packages from nvidia that you may have installed and remove them all.
dpkg -l | grep -i nvidia
sudo apt remove --purge nvidia-*
sudo apt remove --purge libnvidia-*
sudo apt remove --purge libnvidia-*:i386
sudo apt autoremove --purge
except the package nvidia-common, all other packages should be purged. (In my case this package was not available in current nvidia driver, so not removed.)
Search for the xorg.conf file and remove it as it might contain nvidia configuration.
sudo rm /etc/X11/xorg.conf
Remove any blacklisting on nouveau from kernel load (which in my case is as follows:)
sudo rm /etc/modprobe.d/nvidia-installer-disable-nouveau.conf
List available ubuntu drivers for install, and install nouveau.
sudo apt install nvidia-firmware
sudo apt install xserver-xorg-video-nouveau
If you want to be sure that nouveau will be loaded during boot, you can force-load it by adding it to /etc/modules.
echo 'nouveau' | sudo tee -a /etc/modules
If above does not work OK after restart, you can try and also execute
sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx
You may also want to remove Compton and Compiz as they are not very useful.
sudo apt remove --purge compiz compiz-core compiz-mate compiz-plugins compiz-plugins-default libdecoration0
will be removed, also compiz-gnome and libcompizconfig0
Probably this is enough for compton removal, maybe one should search more about this...
sudo apt remove --purge compton