Thanks, Mojca. Inspired by your suggestions, I did the following (which
should work on Ubuntu too, but I am using Debian testing
a.k.a. stretch):
$ dpkg --add-architecture i386
$ apt update
$ apt install libc6-i386
and voila my old binaries from the minimal worked!
There were other issues that aren't so easy to solve:
1. asymptote meanwhile is at version 2.35 instead of 2.31, which affects
figure sizes slightly.
2. graphviz and dot (for all the tree diagrams) were at version 2.26 and
are now at 2.28, which affects figure sizes somewhat and their layout
slightly.
3. Inkscape was 0.48 and is now 0.91. That affected the conversion from
svg to eps of one figure. That I worked around by just using the
previously converted eps figure instead of the svg.
All the changes above can affect the page and line breaks, but I tweaked
a bit until that didn't happen.
It does show a general problem with large complex documents regarding
software versions. ConTeXt is the fastest-changing component, but the
other components also change, and one needs a way to preserve the entire
software stack.
That's a software-engineering problem, and maybe others have found
solutions?
My first book (_Street-Fighting Mathematics_) is frozen with its typos
because I cannot reproduce its stack at all, so all the new printings
get the old typos. Through bad luck, at the time (2009) I happened to
have a never-released version of the tex fonts, now long overwritten by
proper versions, and I also didn't use a ConTeXt minimal back then, so I
don't have the other binaries and TeX in the right form.