Look at how many people claim Alignment was a straight jacket on Monsters, despite it specifically being called out as NOT one, and you may have your answer.
It does seem to be the case: unless the official books spell something out explicitly, it is as if it doesnt exist.
Part of it is, we all know that Rule Zero exists. We distinguish between what the official description says, versus what an individual DM does.
But it even seems it isnt enough to mention the possibility of an alternative. The alternative must be explicitly official.
For example, in the 2014 Monster Manual the intro section officially describes what the Alignment in the monster stat block means. It actually says, the listed alignment is just the "default", and the DM can reassign any alignment to any creature.
"
The alignment specified in a monster's stat block is the default. Feel free to depart from it and change a monster's alignment to suit the needs of your campaign. If you want a good-aligned green dragon or an evil storm giant, there's nothing stopping you.
"
The
mistake here is, this permission sounds too much like Rule Zero − youre the DM so do what you want. That in turn implies, to deviate from the statblock alignment is as if "unofficial". Suddenly, there is a problem with those DMs who play "by the book", and with how WotC officially presents D&D.
The feeling is, as if, the possibility of an alternate alignment
doesnt exist.
Now statblocks clearly − officially − add the word "typically" to many statblock alignments. So, the fact that there exists in the core setting assumption individuals of other alignments, is official.
This need to spell things out officially, is why I sympathize with the fans who want the Human-Elf to be in the 2024 Players Handbook − explicitly and officially. I agree, if there is only the mention of a possibility of a Human-Elf without supplying a clear example of it, the Human-Elf really can disappear from the D&D lore.
I dont think the Human-Elf should count as a separate species. I do think the Human-Elf should be a prominent example of a multispecies in the 2024 Players Handbook. Maybe the 2024 Players Handbook should have two examples of multispecies characters: the Human-Elf because it is extremely popular and a convenient pregen using multispecies rules, and something because it is obscure, such as a Dragonborn-Halfling, to showcase the creative possibilities.
If something is an important part of the D&D tradition, then officialdom needs to spell it out in full.