I've been trying to determine exactly what it is about this "retro" style of anime that makes me like it, and as far as I can tell, it comes down to two things.

1: The shading.
2: The (comparatively) muted colors.

Well, the obvious big difference between then and now is the advent of computer animation, and of course, screen resolution. If you have a NES and a Switch, it's easy to see the difference in video quality between the two on different displays. The NES looks like trash on a modern HDTV, but the NES outclasses it's successor when you use an old CRT. It's probably why a lot of these retro artists apply a filter or effect of some sort to give that "old school analog" look.

And it's been said that limitation breeds creativity, so it might be the creativity spawned from those limitations that make such work appealing.