Hi Kyle,
I’ve connected with our internal teams who have confirmed that application/graphql has never strictly been an allowed request content-type header, however it looks like it was just accepted previously.
The response formats should be ‘application/graphql-response+json’ or ‘application/json’, with ‘application/graphql-response+json’ being preferred as it allows for status codes other than ‘200’.
The changelog for this in 2025-01 is here GraphQL Over HTTP — Shopify developer changelog
And info on the change limiting to json is here: Content type `application/graphql` is deprecated — Shopify developer changelog