This data pack allows data to be stored per entity. Usage:

1. Always run the "entitydb:get" function first:
   execute as (entity) run function entitydb:get
   or:
   data modify storage entitydb: UUID set value (...)
function entitydb:get

2. Do something with the data:
   data modify storage entitydb: entities[-1].data set value {test:1b}

3. Save it:
   function entitydb:save

To remove an entry, remove "entitydb: data" and run the "entitydb:save" function.
The "entitydb:get" function removes "entitydb: UUID" after running.
The "entitydb:save" function removes "entitydb: data" after running.

How it works: The data of an entity is stored in a jukebox at a position that depends on the UUID[0] value.
This allows fast access to the data because it doesn't need to filter through a large list of all the stored values.
(Except if there are a lot of stored entities whose UUID[0]/65536 values are equal to each other, but that's very unlikely because UUID[0] is random.)
The jukeboxes are located in the "entitydb:" dimension in the chunk at 0 0.

Here is an alternative implementation of player-specific nbt storage: PlayerDB