OK, I've seen these three letters grace the side of eighth, ninth, tenth, and eleventh generation F-150's but just found out the package was offered in the 1985 Ranger.
How long has it been around and how has it changed? My '13 has the STX package alongside two other option packages and the 5.0, making it a glorified XL model. A friend has an '18 STX with a ton of options and the 3.5 twin turbo V6.
Now after reading about that Ranger I am curious!

 

Until recently, the STX was a discrete model in between the XL and XLT. Starting in 2016, it's just an appearance package for the XL. Available in Ranger, F-150, and Super Duty.