The MPL V2 contract includes a simple upgradability mechanism to provide a safeguard for unknown future use cases. The chosen pattern for upgradability is the NonTransparentProxy pattern, which has already been audited and used by the existing Globals contract. Due to the contained nature of MPL V2 a decision was made to not import the NTP directly, but rather just copy the functions over, most of them without functionality changes.
Notable changes from the NTP for the MapleTokenProxy are:
The updated constructor logic.
The use of a GLOBALS_SLOT in place of an ADMIN_SLOT because to change the implementation of the NTP a call needs to be scheduled in the MapleGlobals contract.
In order to use the existing ERC20 as an implementation for a proxied contract, some small changes were made to it:
Constructor removal Having a constructor in an implementation contract doesn't make much sense as the actual storage used will be the proxy's one.
Immutable modifier Since the constructor was removed, the next step needed was to change the decimals variable from an immutable to a editable storage variable, to ensure that the proxied contracts could correctly reference it.
Making it abstract The resulting contract was marked as abstract, to make signify that it's incomplete and inheriting contracts need to implement functions to correctly return name, symbol and decimal.
The gist of the difference between the two version of the token can be seen here