Indicated best angle and rate of climb speed change with a change in altitude, converging at the absolute ceiling.

Performance data and speed limitations in most airplane manuals are specified in IAS or CAS. I'm not sure I know what you mean by "airplanes do not fly according to IAS". All airplanes really care about is pressure; they don't "sense" true airspeed at all. Maneuvering speed IAS (Va), for example does not vary with altitude or temperature, since the ability to generate lift is reduced at the same rate and for the same reason that indicated airspeed decreases with altitude, keeping Va (IAS) the same for all reasonable altitudes. Va (TAS) does vary with density. This means the entire V-n diagram is based on IAS.

Please note that I am only taking exception with your apparently unconditional statement, "...airplanes do NOT fly according to IAS...". Whether IAS or TAS is important depends on what speed(s) we are considering, and whether we are designing an airplane, creating performance tables for it, or flying it.

GS means ground speed.

I suppose someone might be able to help more if we know more about your requirements. If you're talking about something large and expensive, I offered a suggestion. If you're talking about something with a 10" wingspan that will fly at 5 mph, I'm afraid I'm no help.

<edit> Changed "unqualified statement" to "unconditional statement".