How to develop widely divergent projects in git?

Question

git

The situation is this. At some stage I joined one existing project. Initially, the development of functionality was carried out in a single repository git , then it became necessary for the new client to greatly reduce the functionality and change many different files, types of connections to different DBMS, etc.
Target projects end up being very different from each other.
In the master branch, the functionality is expanded, and in the private branch of the client, these files should not appear at all.
How to develop and keep track of changes?
Difficult to synchronize changes.

Answer 1

How to develop and keep track of changes?

• Move the common to a separate repository, the different - each in its own repository.
  
• Use flags in the code and write some kind of hellish handler that will automatically cut out the excess.
  

Answer 2

It's not a Git issue, it's an issue with your application's architecture. In a normal implementation, there should be a common core and client-dependent extensions, and for delivery of the client, they should be assembled into a single whole in one way or another.
The way you are doing now is like implementing two different applications. If you want to continue, split the storages as you have already been suggested.