Holy cow, I wrote a book!
This is another leftover from 16-bit Windows.
Back in the Win16 days, string resources were also grouped
into bundles of 16, but the strings were in ANSI, not Unicode,
and the prefix was only an 8-bit value.
And 255 is the largest length you can encode in an 8-bit value.
If your 32-bit DLL contains strings longer than 255 characters,
then 16-bit programs would be unable to read those strings.
It appears to be gone now. Good riddance.