I do not need swap, so explicitly specified swap: none
swap list (optional, default: none)
Now I get an error message with 2024.1.0 :
Invalid config for ‘modbus’ at modbus.yaml, line 11: value must be one of [‘byte’, ‘word’, ‘word_byte’] for dictionary value ‘modbus->0->sensors->0->swap’, got ‘none’, please check the docs at Modbus - Home Assistant