How to safely delete/destroy data from a bricked phone?

Unless you have a means to power the device on and wipe the data properly, you need to physically destroy it. The data lives inside an eMMC chip which is conceptually just a SD card soldered to the motherboard. It will not be damaged by salt water (suggested in another answer) or other means that do not physically break the chip into tiny bits. Unless you understand how it works well enough to judge what would destroy it, just assume most things don't.

Pry the phone open, remove the battery, then destroy the rest of the phone physically in a trash compactor or whatever.