Hang on: The Tee Spring code checks if GA is there and works without it, the website breaks because Ad block replaces GA with a fake object that doesn't have the same API. Perhaps some companies have the resources to check if their websites run with different ad blockers, but expecting websites to work when plugins are replacing and breaking running code is a big ask.
I think TeeSpring has the right idea - they're checking that GA is loaded, if it is, then do stuff with it, if it's not, then don't.
I don't understand how a website can and should be expected to adapt to a browser plugin changing the APIs they depend on out from underneath them.