"These questions", asked by different people, are Can sub-races breed and produce half-subraces? and Which are races, and which are species?, closed as "opinion based" and "unclear what you're asking".
Honestly, I have no idea, how can it be "unclear".
In every particular setting there are finite number of species (which are traditionally called "races"). Some of them can naturally interbreed, others can not. In the end of the day, the questions are:
- What species are able to create hybrid offspring?
- Would be that offspring infertile?
I agree that you can ask this only in context of a particular setting. If the setting wasn't explicitly specified though, the question can be answered like "this setting has various interbreeds, and this one has only half-orcs and half-elves". So these questions are answerable.
However, the only related question that wasn't closed was How could I play a Half-Dwarf Half-Elf?, and the answer was "use the 'Creating a Race' chapter of DMG, page 285", so it actually didn't answer were there dwarf-elf hybrids in the world or not.
Racial traits are base properties of a setting. If you play, let's say, a faerunian setting, you can't just "make up" such important details. There are different species. Some of them are intelligent, others are not. Some of them can interbreed, others can not. There are official sources, describing how are the things going in the world, for this particular setting. I don't understand how can it be "opinion based".
Answers can vary between editions, I thought, so I've asked What D&D editions describe any hybrids of non-human races? This one was closed as "too broad".
How can we improve these questions to make them answerable? Or would it be better to ask another one?