Unless I'm overlooking something, for Ruby users, I don't see other memory controllers that are guaranteed to work. Besides RubyMemoryControl, all others use a QueuedSlavePort for their input queues. Given that Ruby hasn't added complete flow control, PacketQueue size restrictions can be exceeded (triggering the panic). This occurs infrequently/irregularly with aggressive GPUs in gem5-gpu, and appears difficult to fix in a systematic way.
Regardless of the fact we've deprecated RubyMemoryControl, this is a necessary fix.

No memory controller is using QueuedSlaavePort for any input queues. The DRAMCtrl class uses it for the response output queue, that's all. If that is really an issue we can move away from it and enfore an upper bound on responses by not accepting new requests. That said, if we hit the limit I would argue something else is fundamentally broken in the system and should be addressed.
In any case, the discussion whether to remove RubyMemoryControl or not should be completely decoupled.