Yes, there are steelhead in so cal. Mostly small, although really big ones are seen almost every year. Several creeks are "managed" to let them enter and spawn. Several years ago one very large wildfire burned into the back country watershed of a southern creek that held a population of steelhead. Subsequent heavy rains filled the creek with ash and mud, killing everything in it. It would make sense that adult fish, in the sea-run part of their lives at the time of the flood, have since returned to that area to spawn, but I haven't seen any reports of that happening.
There are those who fish for them, and they aren't saying much.