Do smart dimmer switches require a neutral wire?

It depends on the model. Lutron's Caseta and Diva Smart dimmers are notable for working without a neutral wire, which makes them easier to install in older homes. By contrast, most Wi-Fi models such as the TP-Link Kasa, Tapo, Treatlife, and Leviton Decora Smart lines explicitly require a neutral wire, so check your wall box before buying.