The recommended replacement is Keysight's 86207A RF Bridge, 75 Ohm, 300 kHz to 3 GHz
The Keysight 8721A is used in return-loss measurements made with a swept source such as a tracking generator and spectrum analyzer. This 50 ohm bridge (75 ohm optional) has a 6 dB insertion loss and 6 dB coupled to the auxiliary arm.
HP / Agilent 8721A