Lost the Option to Boot into macOS After Windows 10 Bootcamp Installation
If you have access to Windows only I'd suggest you to try these steps:

Access the run command via Win + R (I don't know how Bootcamp actually remaps Win key, alternatively just open start menu and type run).

Insert msconfig once inside the Run GUI. 

Once accessed the System Configuration Utility click on the Boot tab.

Check the option No GUI boot and click OK.

Now you have to Restart and hold down Option key.


You should now see Mac OS and Recovery as boot options.
Enter Recovery --&gt; Startup Disk, now you may be able to select you default boot drive.
If you have Filevault enabled, one way of making the default boot drive selection option appear is to change the Boot Security from High to Medium or None back to High. It will then prompt you to enter your password so it can check to confirm that the encrypted drive is bootable. Then you can select the partition and choose to boot from it.
Now Reboot as you would normally(without pressing any key while you're doing it) and you should boot automatically inside Mac OS, from there I'd create a new Bootcamp instance.