Restricted (hidden) local drives parameters are written into the group policy branch in the user's hive (HKCU). It's located in:
value name: NoDrives
Value is bit-encoded, where 1 - restriced drive and 0 - non-restricted. Lower bits correspond to drives A, B, C, D, etc.
For example, to restrict only drives A, B and C value will be 111 binary. This is 7 decimal.
Please see this link for more detailed explanation:
Explanation of drive restrictions bit-encoding