Replacing old 3-wire line voltage single-pole thermostat with a 2-wire one
The M221sp is a single pole that uses the neutral for a heat anticipator feature, which is really almost pointless with a baseboard. The Honeywell you indicated doesn't have that feature.

After you connect the new stat you will have one less wire in the wire connector with the remaining whites. Sometimes when installers twist the wires so hard that the wires spiral I have found the internal metal cone in the connector becomes flared at the tip and won't thread onto a new connection well. It would be a good idea to replace the wire connectors or at least be very sure that the existing connector makes a good bite, it could be most problematic on the white splice since it is now smaller.