Today I Learned About Multi-Classing into Fighter
Published: April 3, 2020
Today I learned that when you multi-class into fighter, you do not gain proficiency in heavy armor. I had assumed that since fighters get proficency with heavy armor, that multi-classing into a fighter would give you the same proficiency.
On page 164 of the Player's Handbook or in the multi-classing section of DndBeyond.com, you can see that fighters only gain proficency in light and medium armor as well as martial weapons.
Because of this, if you are planning on dipping into fighter for a fighting style or action surge and you plan on wearing heavy armor you are better off starting as a fighter and then switching to your main class. If you don't you will need to pick up a feat to give you heavy armour proficiency.