I do not have a xubuntu handy to check what is/isn't installed/available so you will have to give a few details of what is installed.
I have never known a linux distro "just upgrade" a package without some sort of user intervention. Have you asked about this on a xubuntu forum?
Nothing tells me that xubuntu 18.04 is much different from the base version as far as default packages go, and for Gimp that is 2.8.22
Unless you are using a flatpak/snap/appimage version Gimp 2.10 comes from the otto-kesselgulasch/gimp PPA Did you enable this sometime?
xubuntu used to include synaptic as a package manager (if not worth installing it) and the list of repo's might look like this. https://i.imgur.com/1rXN0fP.jpg
Sometimes possible to force an earlier version: https://i.imgur.com/pOAX0qY.jpg but better to uninstall the Gimp 2.10, disable the PPA. Be thorough with Gimp. Go through the list of packages missing one can lead to 'broken packages' annoyance.
Refresh the sources list and install Gimp 2.8 (which will be the only available version)
I have never known a linux distro "just upgrade" a package without some sort of user intervention. Have you asked about this on a xubuntu forum?
Nothing tells me that xubuntu 18.04 is much different from the base version as far as default packages go, and for Gimp that is 2.8.22
Unless you are using a flatpak/snap/appimage version Gimp 2.10 comes from the otto-kesselgulasch/gimp PPA Did you enable this sometime?
xubuntu used to include synaptic as a package manager (if not worth installing it) and the list of repo's might look like this. https://i.imgur.com/1rXN0fP.jpg
Sometimes possible to force an earlier version: https://i.imgur.com/pOAX0qY.jpg but better to uninstall the Gimp 2.10, disable the PPA. Be thorough with Gimp. Go through the list of packages missing one can lead to 'broken packages' annoyance.
Refresh the sources list and install Gimp 2.8 (which will be the only available version)