An RF filter needs a group-delay or phase-linearity limit when frequency-dependent phase inside the occupied band can spread pulses, degrade EVM, bias ranging or misalign channels. Constant delay can often be calibrated; delay variation across the signal band is the usual distortion term.
Define the exact occupied bandwidth, maximum peak-to-peak delay ripple, calculation aperture, temperature and power state. Do not apply the requirement blindly to the entire amplitude passband: steep skirts naturally increase phase curvature, and the usable phase-linear region may need extra guard band.
The project must supply waveform bandwidth, pulse width or symbol rate, allowed distortion and equalization capability. Verify unwrapped phase and group delay on a calibrated VNA with adequate frequency resolution; compare channels if differential timing matters.