Homebrew is shooting themselves in the foot with their decision to drop support for Mojave. I'm actively looking for another package manager and I suspect there are a number of developers who feel the same way.

Or, as you'd said in a personal email to a Homebrew maintainer, "F*ck you, Homebrew": https://twitter.com/MikeMcQuaid/status/1453326455010697225

I think supporting macOS is challenging for any package manager, but Homebrew is in a tougher position than most w/r/t this decision. Their relatively tight coupling with the base system shrinks their dependency trees (in terms of the count of their own packages in each tree), but it makes supporting multiple versions of macOS more work than it might otherwise be.

(Isolating packages from the base system is also work, of course, and it increases your space/download/compile requirements for getting the first user package installed. It's just that once you've done it, it spares you some of the 'if darwin is version XYZ' stuff.)

"drop support" seems strong. Brew supports Mojave, it just doesn't have bottles for everything and won't make bottles for new things. Bottles are an optimisation surely?

Isn't Mojave not getting security updates anymore? How is it fair to want other people to run a known vulnerable operating system on their build servers just so that you don't have to upgrade?

AFAIK there's quite high chance that Apple may issue a security update from time to time even for older OSes if it's really important.