See the following Community topic post from "edjiang":
"Looks like we’re not able to generate custom providers anymore?"

While the docs do seem to indicate that one can generate a custom auth provider:

It is not supported because:
https://github.com/redwoodjs/redwood/blob/c46033a5229bdeba5d2549466e416400a22ba2b4/packages/cli/src/commands/generate/auth/auth.js#L35
and there is no "custom" template.
While it may be relatively easy to setup the web side, the api side is more complicated with the changes needed for graphql and the additional auth lib.
The custom generator could (and should) still add these to the api side, and stub on the web side.
See the following Community topic post from "edjiang":
While the docs do seem to indicate that one can generate a custom auth provider:
It is not supported because:
https://github.com/redwoodjs/redwood/blob/c46033a5229bdeba5d2549466e416400a22ba2b4/packages/cli/src/commands/generate/auth/auth.js#L35
and there is no "custom" template.
While it may be relatively easy to setup the web side, the api side is more complicated with the changes needed for graphql and the additional auth lib.
The custom generator could (and should) still add these to the api side, and stub on the web side.