In v2 according to msdn it’s valid… it fires fine in v2… but the results are not filtered 
https://server/EpicorERP/api/v2/odata/GLB/Ice.BO.UserCodesSvc/UserCodes?$expand=UDCodes($filter=CodeID eq '1030')&$filter=CodeTypeID eq 'PG_1000'
I guess Epicor doesn’t support it.
https://www.odata.org/getting-started/basic-tutorial/
Also looks like Epicor doesnt support /all either
