Actually the exact specs are 4.5VDC for the 555, and 2VDC for the CMOS 7555. There are some circuits that can't handle the low end voltage on regular 555's because of the Darlington Pair on the output. The 555 Hysteretic Oscillator is such a case, it can't go below 6V and work reliably. There are lots of designs that don't care though, and the 7555 doesn't have this limitation since it goes rail to rail. You can also cheat on the 7555 and go down to 1.5V, but the spec sheet doesn't say this.
555's came out when TTL was king, and that did influence the design.
555's came out when TTL was king, and that did influence the design.
