According to Minecraft 1.21 release notes(https://www.minecraft.net/en-us/article/minecraft-java-edition-1-21 ): 

Ominous Bottles can be found uncommonly in any Vaults, and are dropped by Raid Captains which are defeated outside a Raid

However, raid captains other than pillagers doesn't drop Ominous Bottles when defeated. This is not the same as stated in the release notes.

To reproduce: capture raid captains from pillager outpost/raid, moving them outside the raid(by minecart, boat, /tp command, etc.), then kill them. Only pillager captains drop Ominous Bottles.