Who decides the categories? For example, what would the category say next to a cert for Amazon? Google? Netflix? What would the cert say for Slack (who pivoted from one industry to another)?

Attempting to embed additional visible info in the cert requires some centralized consensus, and trusted validation that certs being provisioned match the consensus. And all of this occurs despite the continued fact that end users do not even look at the certificate information anyways, so we’ve deployed a new central body with a bunch of administrative overhead and not moved the needle on actual security.

The applicant chooses the field of endeavour in their trademark they the applicant think best reflects their identity.

As discussed earlier elsewhere, browser verification markers are outdated - users can and do look at modern verification markers. Google themselves use them in the Play store and for every member of the Chrome team's Twitter accounts.

So Stripe picks “financial services”, and then I set up fake-stripe and I pick... “online transactions”?

Also: how does a “modern” verification marker differ from an “outdated” one? The research seems to show that users are terrible at deciding that something is unsafe based on the absence of a marking, regardless of how new and hip the marking is.

Trademarks are applicable across many named domains - the proposal is for the applicant to pick one for display purposes, however they are not limited to having a monopoly on one field of endeavor.

Short version: it would be very difficult to obtain a trademark for 'Stripe' for 'online transactions'.

As mentioned elsewhere, modern verification markers can be found on Twitter, Whatsapp, Facebook, Apple's App Store and Google Play. They are well recognized and should have interesting test results, if Google actually was interested in testing what effective verification would look like.