Your catalogs do not contain copies of the photo files. They only contain a database of where the files are contained on your computer. So the catalogs themselves take up a relatively small amount of your hard drive.
There is no harm in keeping the prior versions of the program and/or the prior catalogs on your computer. Some people will keep older versions of the program because they have features that have been replaced or deleted. Notably, some folk prefer the older Slideshow creator. The older catalogs also serve as at least a partial backup of tags and albums that you have created.
If your Elements 2020 catalog has all the information that has been properly converted from your prior versions, there is also no danger in your removing either the older programs or their catalogs. However, I would make sure that you have a proper backup of all your valuable photos and catalogs.