Atom push is actually implemented. SWORDv2 (but not v3) is based on Atom Push, and https://en.wikipedia.org/wiki/SWORD_(protocol) https://en.wikipedia.org/wiki/

Atom push is actually implemented. SWORDv2 (but not v3) is based on Atom Push, and https://en.wikipedia.org/wiki/SWORD_(protocol) https://en.wikipedia.org/wiki/SWORD_(protocol) lists a few implementations. My employer maintains a SWORDv2 server, and we have a handful of clients pushing to us; most of them are not listed on that page.