For existing customers it’s still possible to use Auth0.js v7 or direct HTTP calls in order to use delegation; this is required for compatibility reasons as otherwise it would be a breaking change.
However, for new customers the delegation flows were disabled as the goal is to move to flows that are aligned with the API Authorization and OIDC compliance feature set. This means new customers currently don’t have the option to use delegation which is a gap that is identified and that we aim to address so that the same requirements unlocked by delegation can be met using flows aligned with the new feature set. However, at this time I don’t have any definitive timeline to share in relation to that availability.
In conclusion, existing customer still have delegation available in order to not break them and they are still able to use it for scenarios such as the one you mentioned. This will allow them to perform gradual migration to the new flows. For new customers it has been disabled so that they don’t take a dependency on something that will be discontinued it its exact current form. Most of the documentation already calls the attention to situations such as this one, however, the page you mention does not do so; I asked this to be reviewed in order to reduce future confusion.
Update: with regards to your update, delegation is indeed currently used for most of the addons so the constraint applies also to other addons that depend on delegation. As an additional note, both the WS-FED and SAML2 addons should not be affected by this.