Brawl Stars Drop Rate calculated from 99 Brawl Stars Boxes.
As you can see, Mythic Elixir drops are more rare than Legendary Elixir drops. Legendary Elixir drops 1.01% of the time. The Mythic Elixir didn't drop at all in 99 boxes. This is also true for the Mythic and Legendary Brawlers.
Common elixirs dropped at a 36.36% rate. The gap between the common and other tiers widened. Rare Elixirs dropped at a 7.07% rate, Super Elixirs at a 5.05% rate, Epic Elixirs/Legendary Elixirs at a 1.01% rate and Mythic Elixirs at a 0% rate.
The Brawlers drop rates seem to mirror the drop rates for the elixirs. Common Brawlers dropped at a 38.38% rate, Rare Brawlers at a 4.04% rate, Super Rare Brawlers at a 1.01% rate, Epic Brawlers at a 2.02% rate, Legendary Brawlers at a 1.01% rate and Mythic Brawlers at a 0% rate.
Surprisingly, Coin Doublers drop at a 2.02% rate while the coin boosts Drop at a 1.01% rate. Using these coin boosts will help when farming Brawl Boxes.
You can view the original Brawl Stars Reddit post here.
If you have any stats pertaining to Brawl Boxes Drop rate, feel free to share your results in the comment section!