Slack is working hard to prevent this kind of interoperability; the incentive for them is to push ahead on features, and being abstract-able behind bitlbee or libpurple impairs that. They have deprecated the API token (called a "Legacy Token" on their site) that bitlbee uses, and there's a sunset date of May 5 2020.
They used to run an IRC relay, too, but got rid of it years ago.
I firmly believe we're just in a bad place on the whole in terms of IM/voip systems for personal use and org use. Nothing is really good enough. The only ones that live up to expectations are mumble and IRC - the absolute no-nonsense of them leaves expectations low and exceeds them. Had slack taken a more open route and stopped being so org focused it had the potential to do really well (in the "good ecosystem" sense, not in the financial sense as clearly they're doing fine.)
Nothing bridges the gap between personal social use, informal org use (gaming, loose projects), and actual org use. How is it that mumble and teamspeak etc. are still far superior voip clients to literally all other offerings, including the well funded and supposedly mature discord which should be excelling here. Only thing mumble/ts can't do is replace voip telco systems, which in fairness nothing else does either.
Slack can't do voip for shit, discord can't even do notifications, irc can only manage offline message history if you're highly tech savvy or pay for it, whatsapp requires a phone with an active connecion and is clumbsy for ephemeral groups (telegram is fairly similar), gitter doesn't even come in to the picture, matrix has the shittiest public servers in the history of technology, xmpp is esoteric gobbeldygook.
The whole space is a mess.