It's very canon. Solomani sleeper ships were being sent out since at least -2460, which is several decades before their discovery of the jump drive. They were still being done well into the Long Night era (the -700s, as you mentioned); although I'd say that was actually the
end of the sleeper ship era, rather than the beginning.
The earliest ones used sleeper ships simply because that was the most advanced technology available at the time. Later, during the Interstellar Wars period, sleepers were used as a means of maximizing the range of Terran colonization (either to "flank" the
Ziru Sirka along its spinward and trailing edges; or out rimward, as a means of putting as much distance as possible between the Terrans and the Vilani threat). And then, during the Long Night era, sleepers were used either for economic reasons -- or because it (once again) was the only technology available at the time.
The most well-known of these, of course, is the ambitious
ESA project that eventually colonized the Islands Cluster subsectors in Reft Sector. There were a total of over 300,000 colonists that were sent out with that mission. There are also human worlds scattered out past the Great Rift (i.e. the Spinward Marches and beyond) that started out as Solomani sleeper colonies. And there are still a few carefully watched sublight ships that are winding their way through the region.
I don't know much about the rimward sublight expeditions (I'm not sure if much was ever actually said); but there are some extremely isolated Solomani pocket empires deep in the wilderness beyond the Solomani Confederation that look like they might be the fruits of those labors. And no doubt the Aslan and Hivers have had to deal with them in their space, as well.
