The decision to remove the ability to save as an earlier version was deliberate.
I assume it’s to ‘encourage’ you to up grade to the latest version, although recent versions of SketchUp make a new version SKP format that is backwardly compatible.
There is a Ruby based extension by Dezmo that allows backwardly compatibility saving of your newest SKPs…
https://forums.sketchup.com/t/save-to-previous-version/274248/2