External Links Policy
I would be surprised if any major platform allows the placement of links to other websites outside their own sphere of influence.
I am in the process of removing all out-bound links from this website as I do not gain anything from having them. It is also a good opertunity to check that all my pages pass the nu HTML validator and then check to see if the page is in the Google index.
Critical Mass
There must be a critical mass in that sites such as Twitter (now X) where it doesn't matter. X does not need page ranking as the "followers" are going there anyway and the platform "feeds" on linking to other sources so it would not make sense to block external links.