@KR98 Thanks for the question, Pablo Castro explains one way to do this in Cognitive Search using document-level access control in this video:
https://www.youtube.com/watch?v=tW2EA4aZ_YQ&t=578s
That restrictions could be placed on a single index (as per https://learn.microsoft.com/en-us/azure/search/search-securi...) and that security filtering was also possible (Security filters to trim results using Active Directory - Azure Cognitive Search | Microsoft Learn).