E3B's website is still online; the firmware updates are in the Support -> Updates section.
Poseidon 4.5 was indeed the last version, although a few individual class drivers have been updated since then. The RapidRoad driver package has some of them (see Individual Computers' website), the others are on Aminet. Can't recall with certainty what the last version of Luciferin was, but 4.1 sounds right, or close to it.
As to what happened to these developers...
I think Michael Böhmer was getting his doctorate, which kept him understandably busy, although that might be done by now. I recall some sort of message a few years ago that he'd like to get back to the Amiga eventually but just hasn't had the time.
And in an unfortunate but long-standing Amiga tradition, Chris Hodges was getting harassed/trolled/ripped off by a few inconsiderate jerks in the community and decided to call it quits. I guess some people are still in contact with him because Poseidon continues to be available for MorphOS and RapidRoad, but I haven't seen a public message from him in a long time.