Every time I try to open a 20.0 or earlier scene, I get a massive list of warnings in 20.5 Not sure if they are FYI only or does it mean that there will be issues, and I need to do something that is not clear to me...

For example...

I think that means you opened a scene saved with a different version, and those nodes may have different parameters in the new version so they may or may not produce the same results. For best results, you should recreate all of the nodes that have these warnings-especially the Karma render settings node because that had significant changes.