Grosmimi
The famous Grosmimi straw cup is a value-and-performance favorite, but the cup body and lid are PPSU - a high-heat medical-grade plastic - with a silicone straw and silicone sealing valve. So the drink sits in a plastic vessel and the child sips through silicone: minimal-contact, not plastic-free, despite the PPSU-is-safer framing. Grosmimi also makes a stainless-steel straw cup (304 outer / 316 inner) that keeps the drink off plastic - the better pick for avoiding it.
How clear are their specs?
Names PPSU for the plastic cups and 304/316 stainless for the steel cups, and states the straw and sealing valve are platinum silicone - materials are fully disclosed on the product pages.
