Event bonus item drops are not affected by drop rate boost buffs

[The Secret Cow Level]

Total killed with 20% drop rate rune equipped, no prestige: (Hard mode, 2 runs each, minion on autoloot)

• Dukaki Swindler:  472
• Garden Tentaklud: 982
  • Total: 1,454
  • Event coins: 511
  • Event coin drop rate: 35.1%

Total killed without drop rune equipped, no prestige: (Hard mode, 2 runs each, minion on autoloot)

• Dukaki Swindler 472
• Garden Tentaklud 983
  • Total: 1,455
  • Event coins:  506
  • Event coin drop rate: 34.8%

Thus the 20% drop raterune does not increase drop rate of event coins.

Back in the 5.8 end-of-the-world event, I also observed that in the LInkgate instance, server drop rate event also did not affect the drop rate of the event drops.

So if you only have 1 support rune slot, socket the XP rune (if and only if you are not using other XP buffs)

(Maximum number of mobs you can kill in Garden of Growth is 728, not including the harvest plants. At 35% drop rate, you can get about 255 event coins if you are able to kill everything that spawned.)