As I recall (not sure), the prohibition is against a DA20 flying in the clouds due to lack of protection, not to flying on an IFR flight plan.
The old Ordedr 8700.1 is incorporated into ORDER 8900.1 - Flight Standards Information Management System (FSIMS) which is available at
http://fsims.faa.gov Have fun searching; it's huge and covers a lot of different things (as did the okd 8700)