In cost-conscious live sound scenes (ex: contra dance) for decades one monitor mix was standard and two was bonus. Then two was standard. Now it's common for even relatively low end systems to be able to give you more mixes than you need, and in some scenes a separate stereo mix for each each member's in-ears is standard. On the other hand, we generally haven't seen an increase in the number of inputs. What changed? Asymptotic complexity!
With a traditional analog mixing board, a large part of the cost is
proportional to the product of your inputs and your outputs. For each
input you need to decide how much should go to each output, which
means at least a little knob. If a board has N inputs, then every
additional output requires another N knobs.
With a digital board, however, it's all virtual. You need hardware
for each input (N) and each output (M) but
the cost is O(N+M) instead of O(N*M).
In general inputs are critical while outputs are nice-to-have: two band members can much more easily share a monitor mix than a mic. Given the old cost dynamic mixers went for a lot of inputs with a few outputs; now it's cheap enough to throw in a few more outputs that even cheap digital systems like the 1818VSL I got in 2012 will offer things like eight outputs for eight inputs.
(While I like the Big-O explanation, there's also another factor which is that the process of turning an audio signal into sound that you can hear has changed. The traditional way of doing this is an amplifier that drives one or more unpowered speakers. Then amplifiers got smaller, lighter, and cheaper and we integrated them into the speaker boxes. This meant that when you got multiple speakers so you could point one at each person, you were also getting multiple amplifiers and each speaker was able to handle a separate mix if only the board could provide one.)