This will NOT work.
You cannot get
ICVS curves in API. Excel has a curves toolkit for excel. However, for
WAPI, you need enterprise licenses (not part of terminal subscription). Your account manager can help you with that if that is truly needed.
What you do here, is simply a FLDS field. Check out
YCSW0045 Index FLDS CURVE_TENOR_RATES on the terminal. You will see that the field description shows that only current value is available.
Side not, I would say that is not the best way to ask this. Use the help desk. They will most certainly not help with a 3rd party wrapper (I never used this one, but is this only Python 2.7?). However, always ask how this works in excel (it's both c++ according to Bloomberg). I am sure it will not have taken almost 6 months for BBG to answer this.