When google_sign_in_android switches to Credential Manager shortly, it will no longer have an implementation of disconnect, as the current APIs don't yet have an API for this, and continuing to use the long-deprecated APIs would subject the plugin to build breakage in cases where Gradle does optimistic upgrades. There is supposed to be an API coming for this in the future, so we should check back regularly to see if it's become available. (As of filing, the 1.6 alphas don't appear to have it.)

I'm labeling this as a regression since at least in theory it is one. Its unclear whether that's true in practice; #26401 says it didn't work on Android, but that was only reported by one person, and never confirmed. (On the other hand, it's not clear how many developers would actually notice if disconnect didn't work, as it's not necessarily something that people would test.)