Shortfuel: Many contributors here have referred to FCOM 3.04.30 P1 Suppl. Procedures. Under the advice given, in bold outline, Airbus have always said to avoid
"extended flight in icing conditions with slats extended"
Why do you think that is?
Once the gap in the leading edge is created by slat deployment you are not going to get the same heat transfer to the leading edge hence much less protection than when the leading edge is `closed`. This is Airbus admitting to you (warning you perhaps) that you should be aware of the lesser protection. Which now applies to the whole leading edge and not just the non heated areas.
All the types that I have flown with slats had similar considerations.