[v3] Migrating to new ad unit sizes and unifying default banners

[limbofeather]

As a part of the v3 update to Zesty's SDK, we will be migrating from wide, square and tall ad unit sizes to their closest IAB formats billboard, medium rectangle and mobile phone interstitial, respectively. We will also be removing the minimal and transparent ad defaults and unifying them into one.

Below is the mapping:

• wide (1024x256) --> billboard (970x250)
• square (1024x1024) --> medium rectangle (300x250)
• tall (768x1024) --> mobile phone interstitial (640x1136 or 750x1334 or 1080x1920)

Migrating

In order to not break existing ad units by pre-emptively removing the older formats, we will place the new ad formats behind a flag so that only specified spatial apps will have the new mapping enabled.

Gradually over a course of one month we will begin to phase out the older formats by lifting the flag so that all spatial apps that use the Zesty SDK by default use the new ad formats.

[limbofeather]

We'll also need to fully deprecate the tall, wide and square banners.

[limbofeather]

Formally closed by #210