Sites like g2a and such that were apart of those conspiracies are not the same as the sites that sponsor us, the main difference being that sites like G2A and such that were the center of the issues devs had were marketplaces, where anyone could go and sell off keys for whatever they like (criminals included). The sites we sponsor are not marketplaces, all keys are being supplied by one single company on this one site, and are technically legitimate keys.
To tl;dr what FAST said above, the sites that are sponsoring us are key resellers, the way they function is they buy "leftover" volume licensing keys from things like businesses or retailers, usually ones that are going defunct and are looking to make quick cash, for cheapo. They can then supply these keys to users at a heavy discount vs normal price. The keys themselves are, technically speaking, "legitimate" and legal.
However, there are a couple of caveats with this practice. For one, it's against MS's ToS to resell these keys, so in theory MS could sue and shut down any of these sites and revoke any keys they provided to buyers. But it is not an "illegal" practice in that they could go to jail for doing this, much like any other ToS violation. However, Microsoft actually doesn't really give a shit about these practices, mostly because they want everyone to be using Windows 10 (to the point where you can still get it basically for free anyways, despite them ending their "free upgrade" offer years ago), so you'll basically never have any of the things you buy from these sites revoked (except maybe Office keys).
For another, the keys that are supplied are, in 99% of cases, OEM keys, so they cannot be activated on different hardware. If, for example, you activated the key on your gaming PC and a couple years later decided to upgrade the motherboard and CPU, the key may result in being unusable and you'd have to buy another to activate your copy of Windows.
So should you buy these kinds of keys? IMO, probably not, simply because there are legitimate "free" ways to get Windows 10 already (for example, you can literally reuse your own OEM keys from basically any old laptop or desktop from like Windows 7+, if it hasn't already been used to upgrade), and there are good, free alternatives to using MS Office (like LibreOffice or Google Docs) where, IMO, I wouldn't bother buying a copy of it at this point.