Level criteria for getting event drops?

Here is a snippet from the V1.21 update notes -

You must hunt monsters no more than 10 levels lower or 20 levels higher than you for the item to drop.

This is generally the most common range.

V1.24 note -

Collect 100 Jolly Stamps from monsters no more than 11 levels lower or 21 levels higher than you to exchange for 1 Tempest Coin.

V1.27 note -

You receive a collectible to put in each museum when destroying Fishbowls, Crystals, and Trees located throughout the game, or by hunting monsters in your level range (11 levels below to 21 levels above).

V1.28 note -

Most Maple World monsters within 10 levels below or 20 levels above you will drop the sticks.

It's safe to say this is the right level range.