Heh, to indulge you. I will read Sage Advice again.
My interlinear comments are in italic.
Sage Advice
"
What happens if a druid wears metal armor?
The druid explodes.
Well, not actually.
[Humorous intent but actually adds to the confusion.]
Druids have a taboo against wearing metal armor and wielding a metal shield.
[Not in D&D 5e. In 5e there is no taboo.]
The taboo has been part of the class’s story since the class first appeared in
Eldritch Wizardry (1976) and the original
Player’s Handbook (1978).
[The D&D tradition has abandoned many unhelpful flavors and mechanics since 1978.]
The idea is that druids prefer to be protected by animal skins, wood, and other natural materials that aren’t the worked metal that is associated with civilization.
[This idea of a "taboo" is absent from the Players Handbook. Hypothetically interpolating a taboo into 5e, what remains unexplained is, why druids like attacking with metal but not defending with metal.]
Druids don’t lack the ability to wear metal armor.
[Just like Wizards dont lack the ability to wear metal armor.]
They choose not to wear it.
[Which is why the Druid class doesnt grant proficiency with it.]
This choice is part of their identity as a mystical order.
[Different cultures have different mystical orders. Not every setting is Greyhawk.]
Think of it in these terms: a vegetarian can eat meat, but chooses not to.
[However, does a vegetarian like to attack with meat, but avoid defending with meat?]
A druid typically wears
leather,
studded leather, or
hide armor,
[TYPICALLY! There are exceptions.]
and if a druid comes across
scale mail made of a material other than metal, the druid might wear it.
[The Druid is PROFICIENT with non-metal scale mail.]
If you feel strongly about your druid breaking the taboo and donning metal, talk to your DM.
[Your DM might not think that the setting is D&D 1e Greyhawk! Also, it is ok to break the taboo. Also, get the proficiency from elsewhere.]
Each
class has story elements mixed with its game features;
[However, there is no story in the Players Handbook to explain why Druids love metal weapons but not metal shields. Also, not every story is true in every setting, whether a regional setting or a world setting or a cosmological setting.]
the two types of design go hand in hand in D&D, and the story parts are stronger in some classes than in others.
[Stories are important, so need to be coherent, and to avoid conflicting with proficiency rules.]
Druids and paladins have an especially strong dose of story in their design.
[Except, in 5e, there is zero "taboo" in the Druid story. Moreover, the Druid is an earth elementalist who loves metal.]
If you want to depart from your class’s story, your DM has the final say on how far you can go and still be considered a member of the class.
[There is no Druid story in the Players handbook concerning the use of metal armor!]
As long as you abide by your character’s proficiencies,
[Yes, exactly. Follow the proficiency rules that are in the Players Handbook! Get the proficiency from somewhere else.]
you’re not going to break anything in the game system,
[Obviously.]
but you might undermine the story and the world being created in your campaign.
[But there is no story in the Druid class description!!! Most 5e DMs are not "creating a 1e Greyhawk world for their campaigns".]
[Get the metal armor proficiency. Remain 100% in agreement with the stories in the Druid class description!]
[Here is a story. Some Druids love the element of earth! Including metal! And want to be defended by it. There is no contradiction with the Players Handbook.]
[Even if the players want the D&D 1e setting flavor that Sage Advice mentions. It only explains why the Druid lacks metal armor proficiency. A player remains free to gain this proficiency from any other source that grants it.]
"
As I said earlier, the Sage Advice explains little. But it suggests some of the reasoning for why the Druid class doesnt grant proficiency with metal armor. At the same time, to gain the metal armor proficiency from somewhere else is legal, and is how 5e rules work, and agrees with all of the flavors that are in the stories in the Druid class description.