[TeMPOraL]

> Who maintains them?

Developers, probably. But generally, whoever your team/company assigns this task to.

> When product asks engineering to change code => you now also update state charts. Added technical debt.

So is documentation. I would hope a team handles this somehow.

But if you approach it as a problem you actually want to solve, here's an idea: have the canonical statechart in code, and have the visual statechart be generated from that code. Benefits include: your visual/documentation charts are always synchronized with actual code; if somebody starts playing fast and loose with statecharts in code, the picture generator will break - if you treat this as a build error, you've just removed one common source of project rot.