A .skp is little-endian binary whose header holds a UTF-16LE version string. Newer SketchUp reads older files; older SketchUp refuses newer ones. The file itself tells you which side of that rule you are on.
Save As can write down to an older version, which is the answer when a client is still on SketchUp Make. On Windows, a .skb is the auto-save backup of the same model — rename it to .skp to recover a crash.
Where this came from
Notes like this one come out of the file library rather than out of a release schedule. The pages linked above are the entries that were being worked on when the note was written, and those entries are rebuilt with each release of the toolset, so a note can go out of date in a way a format page does not.
When that happens the note stays where it is and gets a correction at the bottom rather than being rewritten, because the point of most of these notes is the order things were tried in. The editors who write them are the ones who maintain the catalogue.