A year ago or so, the ThinkPhone 23 (Snapdragon 8, 2023, a weird "flagship") was available for 229€ new on various retail stores. The phone also supports Mobian/PostmarketOS and the bootloader is unlockable with no adverse effects.

Out of nowhere, it received (along with other older phones) updates up to Android 16.

I wouldn't be surprised if the "sudden" update was just a side effect of Motorola preparing for Graphene to be released on these older phones.