I have nothing to add to the title: Why is the [best-practices] tag not allowed on SO any more?
See the official line which begins:
There are a few tags on Stack Overflow that have bugged me for a long time. Namely:
But I could never quite articulate what, exactly, was wrong with these tags. It's been bothering me more and more as time goes on.
It was definitely a bad decision.
I'm just getting into PHP, and wanted to scan through the PHP questions by the best-practices tag so I could ramp myself up on what's current, but it no longer exists. Now you have to hope the text contains 'best practices'.
It would have been a natural if the tag had existed.