I have a morbid fascination with this and all things social. It baffles me more than it excites.
You are right that more often than not I just get asked to make a particular ratio. 9:16 or 1:1.
If I do get told to make a specific social media deliverable then I will make an effort to consult a spec page. Not specs that the agency thinks they need (4096 x 2304 for LinkedIn!)
One that I have found or gathered myself.
Thanks for sharing those links @finnjaeger
I got a bunch last time from this post: 9x16 framing and other non-standard deliverables