The purpose of a travel system is to be your everyday stroller from birth to toddler and beyond, and so it needs to meet your everyday needs.
While they’re not designed specifically for off-road usage, they can still be used. Plenty of brands use suspension systems to make it easier to push off-road, and some will use air-inflated tyres.
Consider what your everyday needs are to help make your decision. If you live in the city and will only occasionally need to push the stroller off-road, suspension and air-inflated tyres shouldn’t be a huge factor. If you live in the countryside, however, and you’re regularly talking walks on gravel tracks and bumpy terrains, you might need to stump up for a more premium model with better suspension and off-road capabilities.