if you are side slipping during a crosswind landing, you are doing something wrong. side slipping is flying out of balance, in a crosswind landing you are (well should be!) in balance until the flare.
have a think about the dynamics of side slipping to the left in a 172 with 30 deg of flap for a minute. left wing has full lift, including that generated by the flap. the right wing on the other hand, has the inner wing shielded by the fuselage, the part of the wing that creates the most lift (inner, where the flap is) is receiving poor flow, buffeted airflow, and only the outer wing is generating 'clean' lift.
left rudder is applied, and you're out of balance.
i don't think you're too far away from the lower edge of the envelope here, maybe that's why the POH says no sideslips with flap?
and anyway, with 40 deg of flap, why do you need to sideslip? a poorly set up approach is the only reason, and a 172 will dive pretty steep with full flap anyway -they're not a piper cub with no flap.