@penguin42 @Floppy there are multiple *types* of bridges too, just to confuse - plumbed-in and puppeted. The latter requires user account details baked in, so you *have* to run your own homeserver for those, as there's no way to generalize the service without storing many users credentials locally.
Iirc Slack falls into this group, so you'd likely need your own server - but I could be wrong. @matrix can probably clarify :)