I'm convinced LinkedIn demotes anything that links off-platform

Interesting...why do you think that? I was on LI the other day and it struck me that probably half my feed was "look at this thing on this other site". Admittedly, the link was usually under the fold in the comments, which maybe is a cheat code to get around some real or perceived blockage? Not that any sort of user-hostile behavior from LI would surprise me, I just made a mental note of it at the time.