OMG I just realised blockchain based media apps like Peepeth are also examples of a "shared heap";
https://wiki.p2pfoundation.net/Blockchain_Social_Media_Apps
I'm borrowing a phrase there from @cwebber's blog post about BlueSky's ATProtocol architecture. Where she contrasted "shared heap" networks like ATProto with "message passing" networks like email, XMPP and ActivityPub;
https://dustycloud.org/blog/how-decentralized-is-bluesky/
(I think I remember her saying elsewhere that Matrix is a "shared heap" too?).