The way in which clients encrypt and sync items in Standard Notes is defined by a protocol specification.
Standard Notes accounts created before November 2020, that haven't been upgraded, will be managed using version 003 of the protocol.
Upgraded accounts, and those created from November 2020 onward will be using version 004.
Third-party clients and libraries marked as SN version 003 only will only work with those accounts still using version 003.