AP rengar had appeared in the PBE and I got excited to no avail, it was swept under the rug and never publicly released with no explanation. Can anyone answer why this happened and if Riot can look into changing Rengar back to the version where he had AP scaling?
Because AP rengar was broken beyond belief, and pretty retarded.
August actually did say why he removed the AP Rengar stuff:
It wasn't playing very differently from AD Rengar (if at all), and it had the same goals as full lethality: Burst target as fast as possible.
The important part is that apparently, AP Rengar was bursting faster with less counterplay, and he couldn't figure out a way to meaningfully balance it without making it extremely powerful or rather pathetic, so he left it to being 80% AP on his W. However, with Q's AS AA Reset, 
synergy with AP Rengar is still there, and he still does have his AOE burst. It's just not going to be something August wants to directly support anymore.
that video is from january.
