There is a real difference between an analog and digital multiplexer regardless of speed. For an analog mux, whatever voltage you put in you get out, including mid-rail voltages. For a digital mux, any input < VDD/2 causes an output of 0 V, and any input > VDD/2 causes an output of VDD volts.

You can use an analog mux on digital signals, but it probably won’t be as good as a digital one. However, you can’t use a digital mux to switch analog signals, it won’t work.

Edit: the one exception to this is for bidirectional digital signals you may want to use an analog mux, since you don’t know which side may be driving the lines. This would apply to DRAM since the data lines are bidirectional, so probably using an analog mux.