Addressfield Autocomplete

Have you tried it in Backdrop? It looks quite simple and may just work.

It has been suggested in this discussion, https://forum.backdropcms.org/forum-topic/where-make-request-drupal-modu... that you might post a module port request here:

https://github.com/backdrop-ops/contrib/issues (tag it "port request").

There are no guarantees, but I have seen items posted here get ported. 

It looks like it has a dependency on Gmap which doesn't appear to be ported yet, so it's a multi-step port. I think @stpaultim's suggestion to post a port request in the queue is a good place to start.