block.minecraft.illager_banner
To do: Version range needs fixing and testing, as it's obtainable earlier, there is an additional unstackable type
From version 1.14 Pre-Release 2 to 1.14, certain ominous banners would display the name block.minecraft.illager_banner instead of Ominous Banner.[1]
Obtaining
From version 1.14 Pre-Release 2 to 1.14, any newly generated pillager outpost would feature 8 Ominous Banners. Breaking the banners would drop a banner with the name displayed as "block.minecraft.illager_banner" in orange text. These banners retain their name when updating past 1.14 as long as they are in item form.
Variations
Placing a "block.minecraft.illager_banner" banner and then breaking it after 1.14 would drop an item that appears identical to the original version, but stacks separately. This also affects regular Ominous Banners after version 1.14.4 Pre-Release 7[test]. Using a cauldron on a banner that stacks separately removes the uniqueness and allows it to stack normally again. It is also possible to use a cauldron to remove banner layers, and then use a loom to add more, allowing for several thousand more variations.
Notes
From the snapshot that introduced pillager outposts, snapshot 18w48a, through 1.14 Pre-Release 1 all Ominous Banners were named Illager Banner. Unfortunately, when updating past 1.14 Pre-Release 1 all Illager Banners lose their name and are indistinguishable from regular Ominous Banners.
Gallery
All obtainable versions of the banners, not including redyed variants
A redyed banner
An Illager Banner
