I know C++ added it 14 years after D :-) but I'm curious why you say ' is a better choice? I never read the papers proposing the feature, but the _ hasn't caused any problems for us, and I like it better myself.
I use the quote separator for digit grouping on any occasion, i.e. also outside of computer code. There, we usually have non-monospaced fonts, for which the use of underscores is glaringly uglier compared to use of single quotes. I don't know if you took this design decision from somewhere else or not, but it was an inspired decision nevertheless.
Also, I'm glad and honoured to have a comment like yours in my history. Sorry for late reply.
I agree that its helpful and you notice Ive been firmly on the side of everybody stealing it - which is why "pretty much every language has that" rather than "oh, cool, that's a really nice feature and it's interesting to learn where it diffused from" was such an oddly dismissive response to Walter and I enthusing about it.
This wasn't remotely true in 2000 when D was being created. AFAIK D was the first after Ada, and certainly popularized it as I made many presentations including it.
Perl 5.000 was released in 1994 (see https://perl.bot/p/kw3gbl for a demonstration that had the syntax)
Edit: That bot is far too easy to be curious with, and unless I got my testing wrong, perl 2 (from 1988) seems to have been where it was added: https://perl.bot/p/jzyi91
All assistance convincing the rest of the world they should also adopt the feature is very much welcome though :D