Was there any regulation limiting the placement of external vents beyond a certain threshold? Or was it just a gentleman’s agreement between designers?

I don't work in the maritime industry so I might easily be wrong, but my understanding is that these rules are generally enforced not via national laws directly but via insurance. You can build any kind of ship you like, but then you have these classification societies that verify whether a ship has been built to standards, and if not they won't certify it and you'll have a very hard time getting insurance for your vessel. And in the end national laws tend to say things like your ship needs to be insured etc., not directly how it must be constructed.

So a gentleman’s agreement between certain insurance adjusters and actuaries?

The issue with maritime insurance, is that ocean going vessels spend a great deal of time in other countries from where the owner registers the ship, and also time in international waters, which are not governed by the laws of any country.

So you average insurance company is not interested in providing insurance.

If you want to read more about the history of martime insurance, you could start with the history of Lloyds of London.

https://en.wikipedia.org/wiki/Lloyd%27s_of_London

Insurance is by contract and either side can take claims to court if the contract is not upheld. I wouldn’t call that a gentleman’s agreement.

The maritime system is complex. But it wasn’t just made up by a few people. It’s evolved continuously, internationally, for generations.