I run a setup like that on my (outdated) Yosemite machine to provide multiple private TLDs for local deployment/development needs.

I set that up in like 2014? Even back then it was known already that the quick /etc/resolver way was the deprecated way to do things. So I guess they finally killed that feature off?

The proper (more awkward) way is to use scutil directly (which then stores the settings in some binary plist somewhere, I assume).

Maybe try this and see if it still works afterwards?