As you observed the documentation indicates that delegation is not supported in Auth0.js v8. Although whenever possible it’s recommended to upgrade to the latest version if you intend to use delegation for that particular use case the alternatives are either to continue to use v7 for that call or to call the delegation endpoint directly.
The reason for the lack of support in Auth0.js v8 is that most of what the delegation endpoint offered has been superseded by other endpoints/functionality. The current exception is the use case you refer to, but for that you can still make use of v7 which has mentioned in the GitHub repository is still being supported alongside v8.