Unfortunately, maintainability is simply bucketed as a "non-functional" requirement.
Maintainability (and similar NFRs) should actually be considered what preserves and enables the delivery of future functional requirements -- in contrast to framing non-functional requirements as simply "how" the software must do what it does vs. the "what"/functional requirements that "actually matter".
From that standpoint, if a steady flow of features/improvements is important for a project, maintainability isn't really a non-functional requirement at all, and amounts to being a functional requirement, in practice, over anything except the shortest of time horizons.
Unfortunately, maintainability is simply bucketed as a "non-functional" requirement.
Maintainability (and similar NFRs) should actually be considered what preserves and enables the delivery of future functional requirements -- in contrast to framing non-functional requirements as simply "how" the software must do what it does vs. the "what"/functional requirements that "actually matter".
From that standpoint, if a steady flow of features/improvements is important for a project, maintainability isn't really a non-functional requirement at all, and amounts to being a functional requirement, in practice, over anything except the shortest of time horizons.