Not super sure how to tag this, but it's gone on a couple patches now so I figured I ought to report it in case it was slipping under the radar.
For about one patch after Runes Reforged came out, keystone runes were visible in the spectator client just fine. But the patch after that, they disappeared once again... I think this coincided with the HP bar updates, if that might be related.
Pretty easy to reproduce - just spectate a game, and observe the lack of keystones.
Figure that this is pretty important information when you're watching a game, and seeing as it was there for a little while, I'm assuming it not being there is a bug.