Is this something that you have experienced yourself? If so an example would be helpful. Approaching a feeder route and hearing "Maintain... until established" imply two separate situations, to my ear.
From a controller's standpoint, if you are operating on an unpublished route (radar vectors or direct the IAF or similar), we follow the 7110.65 4–8–1b:
b. For aircraft operating on unpublished routes, issue the approach clearance only after the aircraft is:
1. Established on a segment of a published route or instrument approach procedure, or
2. Assigned an altitude to maintain until the aircraft is established on a segment of a published route or instrument approach procedure.
So if you are flying toward the airport but not on any airway or feeder route, you will receive vectors to final, or to an initial fix. Then in accordance with b2 we can say "Maintain 2000 until established on final approach course, cleared VOR runway 18 approach." You descend and maintain 2000, and do not descend any more until you (as PIC) deem yourself established on the published approach segment. (The procedure is slightly different if you are with a Terminal Arrival Area designated for an RNAV approach and above the minimum altitude for that TAA; then we can just say "Cleared RNAV runway 18 approach.")
If, however, you are operating on a published route, such as an airway leading to a feeder route, all we have to say is "Cleared VOR runway 18 approach." You are currently at a safe altitude on the airway, and the approach is specifically designed to give you all the information you need to get from that airway altitude down to 0AGL without running into any terrain. If this involves the use of a feeder route, you are expected to use the feeder route and any altitude restrictions for it.
Essentially: I don't think you would be told "Maintain 2000 until established, cleared approach." Perhaps you would be told "Maintain 2000 until FIXXX, cleared approach" where FIXXX is the beginning of a feeder route and you will intercept FIXXX at an angle of 90º or less; you are then assumed to be established on a published approach segment once you cross FIXXX. Once you are established (i.e. after FIXXX) you may descend in accordance with the published approach procedure.