Mastering Blockchain Programming with Solidity

Avoid removing existing state variables

You should not remove any existing state variables present in the old contract version. This should be avoided. For example, in the following code, the string y state variable has been removed:

//Bad Practicecontract SampleContract {    uint256 public x;    //string public y; //Variable removed / commented}

In any case, if you were to add another variable after removing string y, the new variable would reflect the last value stored in the storage slot.