Recently I was looking to use a 74LS123 dual retriggerable monostable as pulse stretchers. I have used them before in a DTL clock. I only needed a subset of the pins, the /A input for the input pulse and the /Q output for the output pulse. Two additional components, a resistor and a capacitor are needed for each pulse stretcher. Here is the datasheet page with the truth table, only the last 3 rows of the 123 truth table are of interest.

Then it occurred to me that I could use a MCU to do the same job without the R and C. Here's an Arduino simulation:

//
// Emulate half of a 74LS123 in firmware
//

#define NA 2
#define B 3
#define Q 4
#define NQ 5
#define DELAY 1000    // milliseconds

void clear() {
  digitalWrite(Q, LOW);
  digitalWrite(NQ, HIGH);
  digitalWrite(LED_BUILTIN, LOW);
}

void set() {
  digitalWrite(Q, HIGH);
  digitalWrite(NQ, LOW);
  digitalWrite(LED_BUILTIN, HIGH);
}

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(NA, INPUT_PULLUP);
  pinMode(B, INPUT_PULLUP);
  pinMode(Q, OUTPUT);
  pinMode(NQ, OUTPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  clear();
}

void loop() {
  // put your main code here, to run repeatedly:
  byte a, b, trigger;
  do {
    a = digitalRead(NA);
    b = digitalRead(B);
    trigger = (!a == b);
  } while (!trigger);
  //  Serial.println("Triggered");
  set();
  unsigned long later = millis() + DELAY;
  while (millis() < later) {
    a = digitalRead(NA);
    b = digitalRead(B);
    trigger = (!a == b);
    if (trigger) {
      //      Serial.println("Retriggered");
      later = millis() + DELAY;
    }
  }
  //  Serial.println("Off");
  clear();
}

Great, I hear you say, you've just written a firmware equivalent for a circuit, so what?

But what if the MCU is an ultra-cheap one like the 3¢ ones discussed over 3 years ago? That's so cheap that you could admit the audacity of thousands of transistors on a chip substituting for maybe 50 transistors on the monostable chip. You could (re)program the MCU for the duration you want. You might not be able to get very short pulses of the order of microseconds, but on the other hand you could have extremely long durations, days even. And it would be far more accurate than an RC network.

Now such MCUs are certainly cheap enough to be 7-segment decoders and the like, but for 3¢ we can contemplate substituting for even simpler circuit blocks such as switch debouncers. Normally you would fold such functionality into the main MCU's firmware, if the product has one. But you could simplify the firmware by splitting out the fuctionality into another chip.

One thing to note is that in practice you want to read in all the inputs in one instruction, so that you don't get race conditions between /A and B changing. This can be done by reading in a port in one go. You can also ensure that all outputs change at the same time by making sure that the Q and /Q lines are on the same port, although output skew is less of an issue for most applications.

In my application I only need /A and /Q so only two I/O lines are needed. So an 8-pin MCU with 6 I/O lines could support 3 such firmware monostables.