I'm not a CloudSQL user, but In my understanding, you can still use CloudSQL with using the database/sql package.

In Google AppEngine Go 1.9 runtime, we have to use the google.golang.org/appengine package to connect CloudSQL.
In Go 1.11+ runtime, we can use the database/sql package and there is no need to use google.golang.org/appengine package.
#1007 only removes Go 1.9 runtime support. You can still use CloudSQL in Go 1.11+ runtime.

please see migration guide https://cloud.google.com/appengine/docs/standard/go111/go-differences?hl=en

But it's no longer supported through this package, right? Or am I badly misunderstanding things here? 😅

Not right.
This package doesn't support Go 1.9 runtime, and also Google doesn't support it.
In legacy Go 1.9 runtime, Cloud SQL needed the specific package.

But now, Cloud SQL supports MySQL protocol (ref. Cloud SQL for MySQL).
So you can use any driver that supports MySQL protocol (including the go-sql-driver/mysql package).

Oh, okay; sorry for the noise 😅