Get Entitlement Membership

Do we have an (entitlement / any) API to get list of identities who have access to an entitlement ?

You can get that from either search in UI or the v3/search endpoint.

Query via UI would look something like this:

@access(id:{entitlement_id})

Or via POST v3/search

{
  "query": {
    "query": "@access(id:{entitlement_id})"
  },
  "indices": [
    "identities"
  ],
  "includeNested": false,
  "sort": [
    "displayName"
  ]
}