Do you have reasons to work with or on their files, and re-publish the binaries? I believe Stefan is alive and well, so if you want to ask him a question on MUI, I doubt he'll refuse to answer if you ask politely. I know nothing about the whereabouts of Holger, but again, just being too lazy to ask does not mean that you get rights on his programs. Maybe they don't make statements when they go, but why does that stop you asking?
Aside from a little Commodore 64 BASIC and some VB in college, I'm not a coder, so no, I'd have no reason to work on or re-publish their binaries. I was just using them as examples.
Stefan, for example, from multiple forum searches I've found many posts from people who've already tried to contact him. Laugh at me all you want, but when I found
this website this past May I went ahead and sent him the Paypal registration fee. When I was a broke teenager I pirated the heck out of his software. Now it just felt right to "make up for that", in a way, by registering. I got no response at all, not even a "hey, thanks for the money". Which is kind of a shame, but I'm still glad I sent it, in a "doing the right thing" kind of way.
Around the same time I registered the WarpDT datatypes and a few other Amiga software packages that I've used over the years. Again, just trying to do the right thing. Others did write me back, which kind of warmed my heart to know they appreciated the gesture.
Holger, on the other hand, I believe he "flipped the collective bird" to the entire Amiga community some years ago, and dropped off the map. No longer accepting registrations or even willing to discuss his Miami software, assuming you could even track down his contact info. Which is a damned shame, because I would gladly put money in his hands today to see that software continued to be developed.
But I guess for some people, pride is more important than money. I don't know, just hypothesizing.