Complaints should, in the first instance, be raised through the staff/student committee where they are minuted and actioned. However, this may not be appropriate given that the university has taken the decision to suspend the programme and not offer any optional modules. I have worked on programmes that are being phased out and it is a sad state of affairs for all concerned. We are going to see more and more of this in the sector. As long as the university has followed the proper procedures in doing so, you may struggle to build a case, but as it has be suggested, the SU may be able to advise.
Regarding optional modules, there is probably some small print somewhere that says that optional modules may not run every year and what's on offer may change for year to year. Tha's certainly the case where I work.