In Android 11 you'll now see that notification settings and per contact notification settings are now all managed via Android Settings (i.e. no longer managed directly in the Textra user interface).
You can still access these Android Settings via Textra using the Android Settings button or via Android Settings > Apps and notifications > Textra > Notifications or long pressing on an incoming message notification.
Unfortunately, there is no Textra heads-up style anymore, it is not compatible with Android 11 Conversation Style notifications. In addition the Android Settings user interface may or may not provide the LED color option (phone specific) or the vibration pattern (phone specific).
To turn on Android 11 Conversation Bubbles for a given conversation, tap the icon (bottom right) on an incoming message notification (looks like a square with an arrow in it). To turn off conversation bubbles, go to that conversation setting (via Textra, Android or the Notification) and turn off 'Bubble this conversation'.