Well, let’s say that x11 is an unbloated terminal that many people seem to be using. At least all my colleagues and I are using it. Ubuntu-Linux by default uses wxt and all my colleagues and I have a “set term x11” at the start of their .gnuplot. Maybe that’s a dinosaur attitude but when working remotely on servers as many of us do all the bells and whistles of specific terminals pale compared to the speed of x11. And when one uses gnuplot it’s usually not to have a user friendly environment but mostly to have a quick and dirty way to plot. Then one uses python/c++ with whatever package to do the “real work”.
Given as I said earlier the statistics of how gnuplot is usually installed (as seen from homebrew formula) and that most webpages with instructions to install it suggest to add x11, it seems to me this is a reasonable option to add. I understand that if you move away from options (just tried to install opencv with the usual options and it failed, so I guess that’s a new system-wide decision that will impact the hundreds of webpage with help about how to instal xxxx on mac with homebrew, not sure this is a good idea but I’m sure you gave a long thought to that before taking the decision), it means it has to be a new default. I don’t know how many people had gnuplot without xquartz before, maybe that’s the data you need to take the decision. I’d be happy to overwrite my manual installation with the homebrew one for sure if ever x11 is available in it.