It's part of the bridge on FW 5.12 (that's precisely what the "**** 5.12.x

****" message is feedback for).
If it's gone, something is doing sneaky stuff to it, because at a very quick glance at a 5.12.5 image, nothing appears to be openly doing something new about it.
(keyword being "quickly" and "openly". I have no desire/time/motivation to look into this further. There's a giant "stay away from 5.12.x" warning everywhere. Don't be surprised of getting burned when you start playing with fire).