Whitening Matched Filters for Equalizer
You are right that in scenarios where the channel is unknown and/or time-varying, we cannot implement a fixed matched filter. The first filter in the signal chain is usually an analog band pass filter eliminating out-of-band  noise. Then after A/D conversion one would use an adaptive equalizer that performs part of the function of the matched filter, but that will usually also be optimized for further noise suppression. This optimality criterion for equalizers that combines minimization of intersymbol-interference and noise power is commonly referred to as minimum mean-square error (MSE) criterion.