How do I kill entities at y=65?

First, create a scorebaord called "y".

/scoreboard objectives add y dummy

Then, the following commands are:

/execute as @e[tag=test1,type=!player] store result score @s y run data get entity @s Pos[1]
/kill @e[scores={y=65}]

The first one will give the score to the Potions acording to their Y axis and the second one will kill every entity that has their y coordinate to 65. Note that it will only work in that specific hight.

If you wish to kill everything from 65 and above use:

/kill @e[scores={y=65..}]

Side note: The void is actually under the 0 Y axis and not above or in the 65. Not sure what you meant there.