Yes, they still spawn.
Here is the list of systems:
https://docs.google.com/spreadsheets/d/1nbAAtymsXCJib_plQtbjGPkVqlr3VuJxEE95N2n6Phs/edit#gid=0
They are found in SSS free-floating (50ls from main star)
Sorry for the segue, but I spotted this a little while back.
I can't speak for signal-source generation mechanics in general (e.g do SS's spawn more when you're "in proximity" rather than in deep space), but the 50ls rule isn't a hard requirement. I collected many at over 1,000ls using the same techniques I would anywhere. In the early days I used to scan planets while waiting for SSS to spawn, some at over 300k+ ls away, and I'd still get spawns. Granted, as you approach 100c+ velocity, it becomes much harder to "catch" SSS as they very quickly disappear from your 1,000ls scan 'limit'*.
* This may not be an intuitive statement, happy to elaborate if needed, but tl;dr it's why the Voyager probes 'disappeared' (they're only visible when you get within 1000ls).
(Stated upthread, but repeating because it's a ginormous thread...)
My ranks are Expert/Merchant/Pioneer, so those are the upper limit on the mission's rank requirements. I don't remember what the exact requirement was, because I hadn't realised that I'd found a New Thing when I was in the moment of accepting the mission.
Searching the shell is a good idea, I've spent some time there before moving onto Merope.
Awesome, +rep
A bit of an aside, but I'm probably stepping away from UA-related stuff to go back to BGS'ing for a while. Despite the shaky state of missions, I got 3% improvement in two target systems for my chosen factions last night, so I'll be focussing on driving them up for a bit. Plus, with a 17m bounty on my head that didn't get airtime, it's dangerous for me to base out of Kagutsuchi for now.