Read summary. Prior to 18w47a, they didn't have the black border. (Called "Black Bordure" in the tooltip.)
The reason I think this is unintended is due to the fact that Mojang never announced these changes. And don't argue that "Mojang never announces minor changes such as these," as in each snapshot page they clearly have a list of fixed bugs, some of which aren't very major.
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If this is a "new design", then how come the tooltips don't show any black bordure at all?! (Added screenshot)
The banner already had a 7th pattern before 18w47a 😛. Confirmed to have 8 patterns. Added screenshot of result of /data get
For something like this, it's honestly hard to believe that it's not intended. I'd like proof that this isn't a mistake on the developers' part.
I'd also like to point out that Mojang never announced the changes to the Illager Banner. The closest I could find was on the "Minecraft Snapshot 18w47a" page on minecraft.net:
"Changed some block textures based on feedback."
In the third snapshot for minecraft java edition 1.14 they added a thing in which the
illager banners come with the black border. That was the new design for the illager
banner. It is not a bug.