I still don't think that's quite right, since you're presumably numbering the newest item as "0" and then counting up from there. Which means after 20 new posts come in, my link to /#20 no longer takes me where I want it to.
Similarly, with history.pushState and history.replaceState, why use hashtags instead of actual URLs?
I have been thinking about what the url space for infinite scrolling should look like, and I think that if you have meaningful titles you should probably use those as your indicators instead of numbers. Still having paging links makes sense to jump groups of posts.
As for the hashtag, it is a worthwhile thought and you could probably go either way on that one. If you think of the original purpose of the hashtag it was to jump to a specific anchor on the page.
Similarly, with history.pushState and history.replaceState, why use hashtags instead of actual URLs?