It's occurred to me that I've not seen a natural Skeleton Trap spawn in since 1.11.2.

(1.10.2) I could simply fly around and find one nearly every lighting strike due to the high spawn rate. (Each test in conducted in a permanent thunder storm, excluding 1.12.2 where over the course of seven months, no players on private server can vouch for having seen one)

(1.11.2) Even with the considerably lower spawn rate, I managed to find 2 from the 16 strikes.

(1.12.2) I tested this by entering _{/tp @e[type=skeleton_horse]} after nearly every lighting strike on Hard difficulty (using a server world with a naturally reached regional difficulty of 4 (for many many chunks, not just one) over the course of 2000+ in-game days) for about an hour. I found nothing. Same goes for ten minutes in a fresh world- nothing.

Over the course of seven months as noted earlier, no players can vouch for having seen one.

(18w19b) Using _{/tp @e[type=skeleton_horse,limit=1]} I idled about the area and listened. Having observed 80 bolts, not a single Skeleton Trap spawned on Hard with a regional difficulty of 2.25.

UPDATE: I saw one naturally spawn in an ocean in 18w20c on Hard, it's spawn location under a regional difficulty of 2.36. I continue to test.

UPDATE 2: Three spawns, all at night, / Hard / 2.37, in 48 strikes. 
has this been fixed ? O_o

UPDATE 3: Yep. Seems fixed.