I thought about this thread while I was out walking today. Here are a few posts that I saw, signs are of course missing.

This is about halfway up to the ski hut. The sign once was a reminder not to cut switchbacks.

This had a sign saying Baldy Bowl Trail. The post also had a bottle opener attached.

Not very old - this is at the ridge above the rock garden part of the Baldy Bowl trail.
The sign helpfully pointed the way to the trail, rather than having people walk along the ridge.

This is near the Baldy summit. I never saw a sign but I guess it used to mention that it was on the Bear Flat trail.

And this poor remnant had a sign pointing out the beginning of the Devil's Backbone.
The sign at the cutoff up to the register is now gone, although a post is higher on the slope than I remember. Looks like someone just cut the posts rather than unbolting the sign boards.
"Argue for your limitations and sure enough they're yours".
Donald Shimoda