We do not yet have an ETA but we haven't forgotten about it. We've even created a proof of concept.
We have to create a service for listening to websockets connections, monitoring them to ensure they are alive, and kill them when they aren't. This isn't unlike what we do for users, but users aren't expected to be available 24x7 like a bot is, so we can't re-use that infrastructure.
Unfortunately the work is quite different from the SharePoint tab work. In the case of the SharePoint tab, the Teams client initiates the connection; in the case of bots, the Teams service initiates the connection, which is why a bot has to be reachable outside the firewall in the first place.