Wanted to start off simple, as I'm busy with 100 other things.
You're the lucky one to notice the option is missing, Ive been meaning to issue an update but nobody noticed yet lol.
I use strpos because between 3.5 and 3.6 (or something along those lines) they changed that value which broke it on some boards.
I use !$thread['sticky'] because the separator is dependent on the transitions between the different thread types. It should ignore sticky threads or else you'll get a "Closed Issues" header at the very top of your stickies.
My condition is correct. I tested yours and it doesn't work properly.