It’s included by reference as part of the universal plumbing code. There isn’t a “toilet seat act of 1927” or something.

The original intent was to make it easier for women to clean the perineal area without contacting a surface, and reduces genital contact with a surface.

How do I know this? My wife was on a safety committee in a past job where there was a workplace injury claim related to this topic. In our case, there was an automated toilet cover dispenser, which made the round seat code compliant.

> It’s included by reference as part of the universal plumbing code. There isn’t a “toilet seat act of 1927” or something.

Yes. But that standard is being enforced by laws (and perhaps other legal mechanisms) in many places in the US. See https://en.wikipedia.org/wiki/Uniform_Plumbing_Code

I believe it's mostly a matter of hygiene, less contact area for your genitals.