Castoreum is not technically secretions of anal glands, rather they are secretions from castor sacs of beavers which are different from anal glands. Therefore, it is not same as consuming stool of an animal, it belongs to the matters of personal preference whether one wants to consume it or not (Please see: Discussion 73958 • Reply 73992). In my understanding, it is no longer used for vanilla flavouring at an industry-wide scale.

Secondly, in case of any prohibited item included in a product, the principle of chemical transformation would apply when deciding their allowance or prohibition for consumption.

Please also refer to the video below from 14:14 to 15:02

https://youtu.be/ZjQZ39hCY6I?t=854