Instead of this, could we try again to get xindy to work? What was the blocker there, @godbyk?

@tkoeppe If I remember correctly it was an issue the cross-references. Ah, here are my notes.

I reached out to the xindy folks on their mailing list and on TeX's StackExchange, but I didn't get a response.

@tkoeppe, my understanding is that xindy and this change are complementary. In fact, imakeidx can use xindy instead of makeindex by virtue of a configuration option. However, without imakeidx, our LaTeX index production would still be hampered by the 16 streams \write limit, regardless of whether we use makeindex or xindy for the postprocessing.

If the underlying issue is the 16 \write limit, does \usepackage{morewrites} do the trick? (See the morewrites docs for more details.)

"morewrites" is extremely slow. For comparison: "make" after "make clean" with the status quo takes

real 2m56,070s
user 2m50,371s
sys 0m1,175s

Simply adding "morewrites" into std.tex results in this (after a "make clean"):
real 15m32,945s
user 15m2,865s
sys 0m14,112s

That's a factor of five. No, thanks.