I'm upgrading to the next release of Elementary OS and want an easy way to restore all the applications I have installed. While there is a provision for restoring AppCenter purchases documented here, I did not find anything in the Elementary release upgrade guide dealing with free apps.
Ryan M. in the Elementary OS slack community suggested using the 'flatpak list' console command, great idea.
I searched online and found this technique:
1 - Backup all your flatpak apps:
flatpak list --columns=application --app > flatpaks.txt Here you are saving all your installed apps to a text file, without the dependencies/runtimes (they are downloaded automatically anyways).
2 - Install your apps in the new machine/installation:
xargs flatpak install -y < flatpaks.txt
Darshak P. from the slack also informs that "in case you have a separate /home partition, just reusing it in the new installation will keep all user flatpaks intact without you having to do anything."