Taleweaver would indeed be correct, this rule will do absolutely nothing to any normal user using Steam, this is purely for devs/publishers.
This new "rule" Valve is enforcing is supposed to stop game devs/publishers (mainly indie devs, honestly) from abusing the key generation system, effectively creating a "free" Steam game that Valve doesn't receive any cut from.
The main reason this is becoming a thing is because devs generally give streamers and Youtubers a few dozen keys to just give away to viewers, which can obviously cut into Steam's profits quite a bit. For big games that get hundreds of thousands of sales that's not a problem. But when some junky $10 indie game that only sells a few hundred copies starts requesting 1000 keys to give to streamers or Youtubers for them to give away, it becomes a bigger issue. Valve doesn't get a cut from those 1000 keys, effectively "losing money" on that game's listing.