The endpoint /Users/{{id}} does not support filtering.
You got it right to specify the urn:ietf:params:scim:schemas:sailpoint:1.0:User:entitlements in the attributes parameter but you need to filter on the client side.
So, you may be able to get around this instead by searching on the /Accounts endpoint with a filter for the identity ID you are looking for (same one used in your call to GET /Users) and then the ID of the application you are looking at, ex:
http://{{identityiqHost}}/{{identityiqAppName}}/scim/v2/Accounts?filter=identity eq “8a8080824df45873014df45bdd9300dd” and application eq “0a0000017ba71589817e92d46c311b98”
This will return JSON of the Link object associated with the user, as well as the attributes that represent the groups assigned to the user. In this case you’d have to be familiar enough with the application itself to know which attribute indicates group/entitlement.
@adam_creaney - Thanks your suggested workaround worked.
However, I wonder if there is a way to get a filter at /Users working. I tried below as well with no luck
https://{{master}}:12105/identityiq/scim/v2/Users?filter=userName eq "1605124" and urn:ietf:params:scim:schemas:sailpoint:1.0:User:entitlements[application eq "IBM DB2 Database Server"]&attributes=userName,urn:ietf:params:scim:schemas:sailpoint:1.0:User:accounts,urn:ietf:params:scim:schemas:sailpoint:1.0:User:entitlements