for some reason it is still on sketchup official website, but not on the english page.
here it is on the french one : https://help.sketchup.com/fr/downloading-older-versions
or direct link to download :
https://www.sketchup.com/sketchup/2021/SketchUpPro-exe (windows)
https://www.sketchup.com/sketchup/2021/SketchUpPro-dmg (Mac)
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