I was thinking a lot about it as I don't want to put anything illegal on my computer. OTOH, the only way to buy an older version (in my case, Office 2016 for Mac) is to use a reseller. I try to do a research, I try to find a reputable one, I receive an invoice, I install and active the software, it works, but in the end it's as you say - there is no certainty at all.

But in the end I'm fine with that. First, the vendor is no longer selling this product, second, there is only that much one can do.