Shopify has been erroneously returning an elevated number of 429 Too Many Requests status codes for the "/admin/oauth/access_token" used during app install. These requests are necessary for completing an install, so Shopify merchants are occasionally unable to complete the install of new Shopify applications. Gadget's telemetry indicates there is not actually a high number of requests being sent, so we believe this is a bug on Shopify's end and have reported it to them and are awaiting their feedback.
The error is intermittent and comes in bursts, and retries of installs often succeed when attempted ~5 minutes later.
We will update this incident as we learn more.