Hi!
The use of proxy items is still working, as it was one year ago.
There is also transformation support added some months ago, see the docs: https://docs.openhab.org/addons/bindings/modbus1/readme.html (there is example regarding scaling of values)
Furthermore, there is new binding in “review” phase, see Modbus openHAB2 binding available for alpha testing
The new binding is for openhab2, with “things” and openhab2 way of configuring things. I welcome you to try it, if you are brave enough!