Where conscription ends in terms of a useful method for raising an army is when technology becomes sufficiently complex that just not anybody can operate it. There's no point to conscripting large numbers of warm bodies if they are incapable of being trained in a timely manner to perform the duties and tasks necessary to make them useful.
Take an industrial age (or earlier) factory. The tasks any one person was given were relatively simple and repetitive. Learning was fairly quick, almost anyone could manage it, and once learned proficiency could be obtained in a short period of time. Going to an electronics age factory where all the simple, repetitive, tasks are done by machines, you only need a relative handful of highly skilled workers who have been trained over years, to do complex tasks that machines can't do or to supervise the operation of the machines. Hiring a mass of unskilled or marginally skilled workers does nothing for you.
That would mean that starting about TL 8 or so, the need for mass conscription steadily decreases for military operations.
On the other hand, given that some worlds in the Traveller setting are definitely overpopulated, and others are headed there, I'd say one of the likely things for conscription would be colonization and expansion of territory by various space fairing polities. Build a basic ship, pack it with your excess population, and send it off to system X that has had just the most basic survey done to know that there's a planet there they can survive on. Wash, rinse, repeat, until the excess population is no more.
Here, you--the government--doesn't have to particularly care if the "colonists" are suitable for this purpose or not. They're surplus to the government's needs and the cheapest way to get rid of them in some useful fashion is to ship them off to some god forsaken, unclaimed, system to colonize it. You could send criminals, religious zealots, revolutionaries, the disgruntled, persons that don't measure up in terms of productivity or intellect, etc., to do this. They succeed, great! They fail, send another ship load and try again.