The fact that SketchUp 8 has been superceded and not been supported anymore for years is indeed a problem for downloading the official installer. Take in mind that SketchUp 8 also hasn’t received updates to close security vulneribilities in included libraries.
Usually, older versions are officially available here: Downloading older versions | SketchUp Help (only down to 2015). Since it has been dropped from that list, you need to find it on the internet and verify it is not manipulated or infected.
OldVersion.com seem to have only the non-Pro version of SketchUp 8.