there were two remaining occurrences in tex4ht.sty. I changed them also. Hope that was the right thing to do ... closing ... --thanks, karl.

It seems somewhat strange to replace one constant with another constant, thus merely delaying the problem until someone writes a complex-enough luatex document (?), but I guess it's not worth worrying about trying to get the "last open write stream" number out of luatex (only).

Only ... maybe 9999, or 99999, or some even larger number? 1616 doesn't sound far enough away from 16 :). wdyt?

Yay for the macro.

Thanks Karl. It seems that it is intended to print to both the terminal and the log file. It prints for example this message:

::::::::::::::::::::::::::::::::::::::::::
TeX4ht info is available in the log file
::::::::::::::::::::::::::::::::::::::::::

So I think \write-1 is not a good replacement. We should definitely use a macro, defined in tex4ht.sty.

tex4ht uses `\immediate\write16{mesage}` quite a lot. It is potentially dangerous with LuaTeX, because streams with number above 16 may be assigned using \newwrite and tex4ht would then write messages to a file where they don't belong, instead of terminal output.

As a solution, it is possible to use higher number, like `\write1616`.

More information is available here: https://tex.stackexchange.com/a/435226/2891