Times change
Many years ago, post accident, BAe looked at the problem of fuel flowing over the wing from a 'lost' over-wing refuelling cap - HS125. I cannot recall all of the conclusions, but alerts were published re the effect on lateral control.
Similarly there was considerable flight test investigation associated with type IV deicing fluids flowing off the wing inflight - BAe, Canadair, Saab, ATR. (Airbus maintained a watching brief).
Overall, performance was degraded. Some aircraft had performance adjustments after deicing. (ATR changed trim / handling techniques ?)
BAe re-evaluated all Jetstream variants, HS 748, ATP, and 146/RJ aircraft, but as I recall the effects were within the allowed-for certification margins for performance, even with engine failure.
Thus fly the book, don't outthink the situation.
Concerns with deicing performance were somewhat alleviated by other type IV fluid activities; wash-off, application, to address residue dry-out and control freezing.
There was a very old Boeing report re legacy 737 aircraft. Their flight tests indicated that climb performance loss with deicing fluid was up to 6%, but I don't recall any adjustments being required.
Distant memories also recall discussion on heavy rain (Boeing) - but conclusions evade me. However, noting that other manufacturers didn't get too excited by this perhaps it was not a significant problem - margins as above.
But times change; new supercritical wing profiles, sensitive to the merest bug contaminant, similar with engine blades.
Climate change, more intense rain; recall the 737 forced landing when rain put the 'engine-fires' out.