I tried to spawn in a skeleton trap by hitting horses with lightning. I did not succeed with a trident with the channeling enchanment. Neither did I succeed with the /summon command to summon lightning.

The reason why I believe this is a issue, is because I read on the Minecraft wiki that:

"Skeleton horses spawn only from "skeleton traps", when lightning strikes a horse within 4 blocks." (source)

In the history at the bottom of that page, it does never say this was added as a feature. However I believe that this was something that happened in some version, or it would not be written by someone in this wiki.