I propose that guide.proxyChannels be turned OFF by default in the biped and quadruped templates. It does seem to be turned off by default when creating a new custom Shifter rig.
We’ve seen some of the bugs with Proxy Attributes. Some of those bugs with animation layers might be fixed by now.
But there are other problems for animators. When you use Proxy Attributes, in the graph editor, if you have the foot selected, for example, you see a huge list of nested attributes related to all the controllers that also have proxy attributes on them. Here are some screenshots shown with just the IK foot selected. (Maya 2018 and 2019.)
Left: With proxy attributes (This is cropped. It goes down even further! Notice how there are nested lists of Rotate Order attrs!)
Right: Without proxy attributes (It just shows the attrs on this control.)
I think new users are especially going to use the biped template as a starting point. And Proxy Attributes are full of bugs and usability issues, as we’ve seen. I suggest we avoid them by default.