Does the FaceTime eavesdropping bug affect iOS11 devices?
There are no public vulnerabilities for iOS 11 being affected by this specific bug or any of similar function or scope.

As far as the bug from late January on iOS 12 - Group FaceTime is disabled globally since 1/28 so no devices are vulnerable.

Before the service was suspended, only devices with Group FaceTime capabilities could be exploited or considered vulnerable.

Devices on iOS 11 were not affected and won’t be until/unless that feature is added to an update to iOS 11. 

Looking back, the steps to reproduce the bug involved dialing a contact via FaceTime, and before they picked up, dialing a second contact to initiate a Group FaceTime. So, anyone can convince themselves that the issue is mitigated and also test once the service presumably gets re-enabled on patched devices.