I tend to think of the words as staking a position vs its opposite, so you at least have some idea where the product lies on at least one axis.

- minimal vs maximal: "we favor fewer few design elements vs more"

- streamlined vs not streamlined: "we have intentially chosen to focus on a few specific goals"

- sleek vs rough: "we are aiming for a polished appearance"

- simple vs complex: "we prioritize ease of use over supporting lots of use cases"

- opinionated vs configurable: "we have a philosophy of the right way to do this, which this tool supports out of the box; if you have a very specific use case, it might not be for you"

etc..

On the other hand, the opposite of modern is... old fashioned? Isn't almost everything basically modern, then? And how is that necessarily any better?