What is the real daily pin ceiling before Pinterest throttles you?

There is no number. Pinterest has never published one and anyone who tells you 25 is the magic line is repeating a 2019 Tailwind blog post.

What I've seen matter more: how many of those 42 were genuinely new images vs the same graphic recycled onto board 4, 5 and 6. I've run 30+/day for eight months with zero problems because every one is a distinct image. The moment I let a repin loop run, things got weird.

Counterpoint, your drop sounds like the normal distribution reshuffle, not a throttle. Mine did the same in June, no volume change at all, and crawled back over about five weeks. Before you blame the scheduler, check whether your outbound clicks fell by the same ratio as impressions. If clicks held and impressions tanked it's distribution. If both fell off a cliff together, something is up with the pins themselves.

42 is not a lot. I'd be more suspicious of the spacing than the count.

I asked this in the business community two years ago and got a community manager reply that amounted to "post as much quality content as you can consistently." So, no number from them either. What I do now is ramp: never increase daily volume more than ~20% week over week. Whether that's real or superstition I genuinely cannot tell you, but I've stopped having months like the one you're describing.