Note that itsme is not a requirement for anything, so the smartphoneless still have alternatives available. To file your taxes in Belgium, you have the option to file on paper, or online by logging in through one of: eID reader, itsme, 2FA using an app (e.g. Google Auth, Authy), e-mail, text message, or a EU-wide ID system. Who exactly is being excluded here? Also, which other smartphone OSes do you expect them to support?