As I said in the How to test CAA & iodef notification functionality thread, Let's Encrypt doesn't send iodef notifications if a CAA record blocks issuance. There have been a couple issues in Boulder discussing doing so (#1248, #2580), which seem to be closed as something that isn't likely to happen in the near future.
But of course what matters isn't just what Let's Encrypt is doing, but what other CAs are doing too. (That is, if I allow Let's Encrypt to issue for my domains through CAA, the point is to stop unauthorized issuance from other CAs, and perhaps through iodef to get notified when someone is trying to do so.)
So, here's my question that I'm not sure how to research, so I'm posting here: Do "most" CAs send iodef notifications yet? Or is Let's Encrypt being like most of the CA industry in not yet supporting them?
The only information I've been able to find is this paper with data from late 2017, which says that they didn't receive any iodef notifications at all. But that was shortly after CAA was implemented, and perhaps the industry has started to include them?