1.16.2
Learned mobs give equipment without nbt , E.g : zombie give full durable sword, vindicator give white banner.
On first learn mobs always give same nbt item, E.g skeleton give same durable bow.

I'll need to think about how I want to handle this. NBT causes issues with the loot learning as it just causes a huge number of variants of the same drops. I'll see if I can do something for the vanilla mobs in terms of random durability etc.

I have an idea, hook loot table, get item NBT, merge learn table. but "merge priority" need think, if loot table and learn table have same item, who replace who?
and some questions:
5k mobs simulation not enough, need 10k.
I remove end_dragon form customDropsOnly but it not work, in game still use customDrops.
What is the fastest simulation speed？

The mob simulation and speed is configurable via the config files.
(mobSampleSize, tickRate, simTickRate)

CustomDropsOnly means simulate only never try to learn that specific mob.
To remove the end_dragon drops you need to provide your own custom drops file that will override the default internal ones.

Is simTickRate high is faster ? and how custom the drops file? edit the loot.json?

I have an new idea. Add a new setting, simulation loot per loot spawning and use the simulation results spawn loots. But this setting should be disable default. This should not require major code changes.