While I don't see any evidence of a crackdown, bringing unreleased mons online is as stupid as pirating SM and using GTS when it's not out.
They probably can monitor traffic pre-HOME release, but once HOME is out, as there millions of users worldwide, it's unlikely it'll stick out like a sore thumb. Plus, talking about duping, it probably is hard to monitor when there's so many PID variation out there (and that is ignoring the fact that some mons are natural clones. For example, some in-game trades have fixed PID. They can't possibly come up with a comprehensive list. They can't fault everyone...)
Except we can't prove if there's any truth to that.
For example, Pokémon HOME allows Eternal Floette, something that was never ever distributed, to be transferred in.
Wouldn't this be a red flag, and an easy thing to stop?
While not related to HOME entirely, but related to trades, maliciously hacked a certain mon to cause the game to crash.
Yet, this isn't stopped either.
With these extreme examples in mind, it's hard to concretely prove there's proper hack checks.
Yes, there is a limited server side check, but it's not reliable. It's stopping Shiny Solgaleo and Shiny Lunala from being used online, even though it was legitimately received from the event...
Well if anyone did, and they feel like it, they'll tell you.