For what it's worth, I was reading the TNG writer guide, and it suggests that shuttlecraft are meant to be sublight only, presumably so as to not undermine the Enterprise's FTL abilities in stories and make the two ship types have distinct strengths and weaknesses (Enterprise can't land on a planet, but can go fast, Shuttles are slow but smaller and can land & ect). Presumably because it's in the writers guide, that's why warp shuttles aren't meant to be a thing in
In TNG, the large shuttlecraft were explicitly warp-capable. Only the smaller shuttlepods were impulse-only.