Thanks for the in-depth explanation, and for experimenting to see how xetex and friends handle this.
Well I made a few tests also with pdftex and xetex (which has a similar value \XeTeXdashbreakstate) and there is no "like every other engine" behaviour". They all have slight differences how they handle this. If you want to try check the document below.
And this stuff is not simple[...]
While I understand that this is annoying: luatex and the fontloader code is still a moving target, and such changes can not always be avoided[...]
I appreciate that this is a _very_ complicated task, that one shouldn't expect {lua,xe,pdf}tex to work identically, and that these projects are under active development. With that said, I respectfully point out that "--- gives an em dash" is prescribed in almost every TeX and LaTeX reference you can find, including Knuth's TeXbook, Lamport's LaTeX: ADPS, and popular online guides. A change like this is sure to cause a mismatch in expectations for most folks: anybody who doesn't actively follow TeX development on the mailing lists (which I presume is most users) will likely be surprised and frustrated that this decades-old convention suddenly stops working after recent updates.
so you could ask on the context list, if they change it, we will pick it up.
I'll try that, thanks. P.S. I wasn't CC'd on your latest response (and I'm not subscribed to dev-luatex), so I'm replying to the last one I got. Apologies if this mixes up the order in the mailing list a bit.