Wouldn't ipfs be a good fit for this?
If an IPFS node/server can be literally hosted via a Firefox Extension[1], how has nobody done the same for an arguably easier protocol like Activity Pub?
Furthermore, IPFS is too slow to be usable in practice. I'd love it to become a serious alternative to the web in terms of documents and blog posts, but the amount of timeouts and 30+ second loading times you need to tolerate to get any IPFS page to load makes it very difficult to tolerate.
At least with the NFT scams IPFS found a real-life purpose, something I hadn't expected to see. That's dying down now, I think, so who knows how long the most active pin servers will stick around.
The performance of ipfs gateways has improved a lot lately, so even without a native ipfs:// protocol handler that could work nicely in clients.