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In This question there is discussion about how to determine when to start the descent from cruise. There are several factors at play and it is usually calculated for maximum fuel performance.
Would this calculation not be done ahead of time, especially for aircraft cruising in class A airspace (presumably by dispatch for part 121 ops)? Why is this not included in the filed flight plan? Wouldn't ATC want to know when to expect this, especially if you lose coms?
Because the Top of Descent (TOD) is calculated in flight based on current weather data, STAR/Transition assigned, directs received during the cruise portion of the flight, vectors received taking you off your planned route, etc.. The FMC does the calculation for you with higher precision in flight.
For regular ops, ATC can use the rule of thumb 10.000ft / 30 NM to calculate an estimated TOD. In most cases, they will not need to do that however, as they can give instructions to aircraft to plan a certain STAR entry gate at an assigned flightlevel/altitude.
R: DLH123, when ready descend FL240 to be level\to reach at ARPEG
During lost comms, you are expected to maintain your filed or last assigned flightlevel/altitude until you reach the clearance limit. From there you will begin your lost comms procedures which usually include holding at the Initial Approach Fix (IAF), descending at the IAF to the approach altitude, and commending the standard approach procedure for the active runway.
You can calculate ToD beforehand or let the FMS do the work. In reality though, you generally don't get to wait until your optimal ToD to start descending, especially flying into busy airports. If the airport has arrival routes, you'll be flying one and there will be crossing restrictions that are either part of the arrival clearance or expected clearances from ATC.
When you are flying the route you'll set the crossing restriction altitudes in the FMS and the FMS will generate ToD points to make those restrictions. You can also use the 1000' / 3NM rule to calculate your own ToD for each crossing restriction.
Going into smaller fields I often set my own target altitude of 10,000' 30 NM from the airport and used that to generate a ToD for descending from cruise. If you are landing in the direction of your arrival into a small airport and ATC is generous with pilot discretion descents you can sometimes manage an idle descent from cruise all the way to the FAF, but not often.
Finally, while the ToD might not explicitly be on the flight plan, it is accounted for in the planning. The dispatcher has to account for expected fuel burn for the flight so we have the right amount of fuel, and the performance calculations they do takes the descent from cruise into the terminal environment into account.
