From what I understand One plus 7 pro does support analog output through an USB C to 3.5mm adapter. So, a normal adapter without a DAC in it can use One plus 7 pro inbuilt HIFI dac to output sound. This being said OP also support digital output which can be used by any adapter with inbuilt DAC. To be frank if you are not into driving very sensitive or power hungry headphone then go with a normal adapter that costs 2 to 4 dollars which should do the trick.
P.S - This will be same for OnePlus 6T, OnePlus 7, OnePlus 7 Pro, OnePlus 7T, OnePlus 7T Pro. Even OP sells their own USB C to 3.5mm adapter without a DAC for this reason, so the internal DAC will do the trick and send the analog out without any external DAC interference.