first the easy one, yes Strength, Agility and Intellect are only found on keystones. So those aren't so useful really in the drop downs as a keystone is already specific to one hero when it's stats are rolled. I've considered removing those, but hadn't yet.
the "o6" type designations are a new thing I'm trying to add. It's a rough suggestion for when those become OBSOLETE. Meaning the stats found on equipment at those rarities are vastly more significant than what can be gained from runes. So you can kinda plan ahead if it's a rune worth investing in long term, or something to use short term if you are below that threshold.