for a very long period of D&D's history (two editions, three editions if you're going with just the Great Wheel lore rather than PS in specific) it was the gold standard for planar lore, being the assumed default except when a campaign setting chose to specifically have a different take on things (like Eberron's distinct cosmology). We had a large body of common planar lore and assumptions for most of the game's history with the Great Wheel starting in 1e, with Planescape being not so much a distinct setting separate from everything else than as a product line specifically exploring the D&D cosmology.
I can't agree with this.
Planescape is not an "elaboration" on the planar appendix of the PHB, or even on the AD&D MoP. It's a whole lot of lore poured into what was, up to that point, largely an empty vessel.
Nor was Planescape the "gold standard" for planar lore. I GMed AD&D from 1984 to 1989, and got Jeff Grubb's MoP as soon as it was available in Australia, and none of that treated Planescape as the default. And I used 2nd ed AD&D material too, in my Rolemaster campaigns, including Greyhawk material and Oriental Adventures material, and none of that treated Planescape as the default.
You don't have to go into crazy detail in the 5e MM, but you should approach each monster with a deeply informed perspective on what has come before and write in such a way as to minimize contradictions. To me that's just being professional, and as I view it, if you're getting paid to write the material, you're getting paid to know it as well as you reasonably can. It's a point of professional pride for me to know all of that stuff and know it well
For me, professionalism in writing RPG fictional content is not primarily about canon and continuity. It's about creativity, and a sense of how engaging ideas can be presented as useable in actual play.
In the period after WotC took over TSR, Ryan Dancey had this to say about "canon" (RPG.DnD.Greyhawk, Sun, 26 Dec 1999 17:45:00):
For a long time, there was an effort to have "one cannon"...
Starting with 3e, we are changing our definiton of cannon. We are going to be moving to an idea called "core continuity"...
The core continuity material is not encyclopedic. We are not going to go through every published product, extract every fact, try to create logical explanations for all the discrepencies, then ask designers to adhere to that mass of data. The "core continuity" will be much smaller - an abstract of the total data, hitting just the most important features...
[T]he amount of knowledge that will be considered "cannon" has to be of a reasonably minimal size. It is simply impossible to keep every piece of fact accurate and checked when the volume of such material expands to the size of something like one of our popular campaign worlds. Trying to do so has created false expectations in the consumer population, and triggered numerous conflicts within the company...
ome of the material produced for our worlds is crap. Pulling no punches, not every word written under the banner of a D&D world logo is suitable for print or should ever have been published. Rather than hold our noses and pretend that such material is signficant, we're going to simply pretend that it does not exist and stop trying to patch it up or fix it.
Good material is more important than canon and continuity. I think this is especially so when we consider new players - they might take up the game because its material is good; they are not going to take up the game because it is adhering to continuity established in a product they have never even heard of, let alone played.
