Mid-era Fenders (and others) featured two discrete preamp channels, one with reverb/vibrato, one without. You could plug into one, or the other, but could not switch between them
Here again, many amp designers began modifying stock amps in various ways, for example, rewiring the preamp stages in series, or voicing them differently and allowing automated channel switching from a single input.
Channel–switching amps are the rule these days, with many featuring as many as six channels, mediated by computerized switching, etc. The results are highly complex, difficult to set up and maintain, and nearly impossible to repair. Moreover, the various channels are more or less minor variations on a single architecture: typically multiple preamp stages, followed by a master volume, before or after the phase inverter.
This design topology/philosophy has numerous implications.