As tiger79 says it is bonded to the DC electric circuits in accordance with the Recreational Craft Directive standards. I have the same on my 2015 boat. It erodes very slowly - even though it is much smaller than the one on your boat. I have replaced it once after the first 3 years in the water all the time.
The saildrive as you know has its own anode and there is no need for anodes for anything else, except perhaps for the steering system which is a mixture of aluminium and stainless components. However there are rarely problems provided you don't use copper based antifouling close to where the rudder stock enters the hull. It is advisable to leave a 100mm or so wide area around the stock and use a copper free AF such as Trilux (same as used for the saildrive housing) in this area.
