Homebrew generally only provides compiled packages for three macOS versions. Now that there is mojave, it means support for 10.11 El Capitan is being dropped. That is why the travis builds are failing.
Unfortunately, I don't have time to do the necessary work. If there is anyone here that does have time and wants to take on the task, I can give instructions on what exactly needs to be done. It will require some development work because last time I checked, qtwebkit needed patching to build on Mojave (maybe a simple thing, I didn't look into it).
From my experience most mac users keep their systems updated. I'm rather worried of FreeCAD soon not working on current OS'es. When Apple drops OpenGL support and requires apps to get "app notarization".
I'm not a dev, but if I can help testing, I'd be glad to. Also I'd like to contribute where I can if someone decides to create a web frontend
not sure if it is what is wanted but i have been able to build qtwebkit 5.9.1 (For qt 5.9.7) on mojave without any issues or needing to patch, (was not for freecad but for other applications so i am unsure if i was using all the options expected for freecad)