This is the response I got from an email to Film Photography Project:
"In our experience, the most important key to long life with E6 is good washing. Formaldehyde used to be used but was discontinued when it was identified as a carcinogen. Hexamine is used in our
original C41 kit. (Not in our new and
recommended c-41/ecn2 kit)
This is a quite unexpected response. The main claim to fame for Heamine is that it decomposes to Formaldehyde (and Ammonia) in water, so I have no idea, how a Hexamine solution could be any less carcinogenic than a similarly concentrated Formaline solution.
The biggest practical difference appears to be not so much the actual toxicity of final rinse, but the regulatory bodies who govern sales of chemistry to end users. It's the same nonsensical thinking as "We can't sell you Acetic Acid, because it's an acid, and that's bad for you, but why don't you buy Sodium Bisulfate instead? It's safe, because it's a salt and not an acid.".
If electronics industry was regulated like that, Thomas J. Watson's alleged quote would have actually been prophetic :-(
I have never seen stabilizer used in E6 kits. Even the current Kodak E6 formula for the final rinse does not have a stabiliser. The basic ingredients are biocides to preserve the solution and wetting agents to prevent spots.
I am quite sure, that Tetenal's final rinse contained either Formalin or Hexamine. You will not find either in 7-bath-kits, because they contain at least one of these compounds in their prebleach.
E6 films have improved stability from previous times. they generally will not fade in dark storage as long as they are properly washed during processing."
There's a chance, that both Kodak and Fuji quietly changed their E-6 couplers to no longer require Formalin, but they did not announce this anywhere I would read about this. There were always kits around with no final rinse, even when Kodak was quite clear that Formalin was needed, so some kits leaving out the Formalin is not really proof of anything IMHO.