Were there modular adventures? I remember Keep on the Borderlands being relatively linear, your goal is to find the big treasure, right?
Modular/module = can put into an ongoing larger campaign.
No, Keep on the Borderlands is not linear and does not have one big treasure as the goal.
A module could be linear, some of the later B series were quite linear. But you don't really see true linearity until late 2e era.