I feel this is a double-edged sword somewhat.
Not including all the playable content in an official traditional setting tells a group of people to stuff themselves.
Including all the playable content in a revised traditional setting tells a group of people to stuff themselves.
(Edit: forgot to respond to this part by itself)
The problem is that you have spoken of only two things: including, or not including. But there are at least three other options:
excluding ("no, we will not ever add this and it emphatically is not part"),
offering ("you could allow X, perhaps by <method Y>..."), and
discouraging ("well, maybe you could have X...but we'll make it suck the entire time and hope you stop.")
Offering might also be called "passive including." Passively including a lot of options is very easy and can be done in nearly any setting. Active inclusion is a bigger ask, I certainly grant that. It'd be cool to see more settings that actively include dragonborn, but frankly I hold out little hope for that. Passive inclusion is nearly trivial--just (a) don't forbid it and (b) offer maybe a paragraph or two somewhere that talks about ideas one could use for broader inclusivity.
That mandates nothing from the people who get their knickers in a twist at the idea of scalybois with extreme halitosis adventuring in the Flanaess. They can always do as noted below: tell anyone who asks that that just isn't an option, please move along if you want it.
Is it easier for a table to add or subtract playable content for a setting in your opinion?
Unless it's load-bearing content, as in critically-important historical events, people, etc. depend on it, it is 100% always easier to remove an option than it is to add one. Just tell people they aren't allowed to play it. Done like dinner.
These boards are full of THACO = bad, and an additive system = good.
Well sure. That's because we literally have actual, scientific research showing that people are better at additive math than subtractive math; that people are better with relatively small additions rather than relatively large additions; and that people are better with round numbers than any old random number. THAC0 is just about the worst of all worlds, since it uses subtractive math, quickly hits multiple digits or (much, MUCH worse)
negative digits, and is completely haphazard in how "bonus" vs "penalty" is described. For real: every combination of "+N bonus," "-N bonus," "-N penalty," and "+N penalty" was used more than once in 2e rules. That's how utterly awful the structure of THAC0 was.
If--and I stress this as an extremely strong IF
--the designers had actually stuck to AC that never became negative and THAC0 that never got beyond a pretty heavily restricted range,
and they actually made it "-5 Vorpal Sword" and "-3 Full Plate" etc., then THAC0 would still have been inferior, but it would have only been somewhat so. But no, they went the whole nine yards for every possible inferiority except requiring division. Other parts of the 2e rules made sure to handle
that stuff.