it was on the Advertising page under the section "Subscription based advertising":

Sponsors and Supporters are able to take advantage of advertising impressions included with their intent to support the site. This is offered as an extra incentive to encourage supporting the guild which runs primarily on donations as opposed to the minimal advertising. The banners/ads could be simply links to their personal blog or website, or any other link or banner that the sponsor or supporter desires. Contact us (see below) for details.

Sponsors (90 dollars per year or 8 dollars per month) can get 15,000 impressions per month on a run of site campaign for the duration of the sponsorship.

Supporters (30 dollars per year or 3 dollars per month) can get 5,000 impressions per month on a run of site campaign for the duration of the support.
I commissioned someone for a banner ad in 2021 since i qualified for either the supporter and sponsor incentives so I was confused it was never added to the rotation? if it's no longer available as an incentive, that's okay too!