Yes a 400 is possible for example when the PUID isn't in the right format. However, that is irrelevant when PUIDs are removed from MB and mostly broken everywhere else (no new PUIDs generated etc.).

So I don't see why we should make a request just for that. Since WS Requests are rate limited, this would slow down without any actual benefits.

The whole point of the deprecatio is to give a bit of notice before we remove PUIDs altogether on our end, too.
Yes, we stop checking PUIDs syntax already. I think that is fine.