Looks like this is a new feature contributed by @mbcahyono, which comes disabled by default.
Looks like it expects you to edit the text in the key js.patreon.donation_prompt.body using Admin > Customize > Text after enabling it, so you can add the link to your Patreon page.
@mbcahyono could we add a site setting with the Patreon campaign URL and make it required before turning on this banner?