Can I still purchase SU 2018 licenses? We have an office full of SU 2018 and we don’t want to upgrade to 2020 or all the licenses have to be upgraded to be compatible.
I don’t know, but if some people use 2020 you could use Eneroth Legacy Save in your daily workflow to more easily save back to the 2018 format. It works basically like save, but if the file is already saved it honors its version. Generally software improves and I think it is worth locking into ways to mitigate friction instead of keep sticking to an old version.
You cannot buy an older version within the store, you would need to contac sales directly, I guess. In General, SketchUp supports the latest three versions. That means, if you have a good reason for an earlier version, the one you buy now (2020) can be downgraded to 2018.
Somehow, you can still download 2017, so I guess it supports four versions, now
I suppose 2017 is kept on the download page as it is the last free version for non-commercial use. If it was up to me 2017 could stay on the download page even when 2018, 2019 and so on are taken down for no longer being supported.