Mark unused variables and functions in engine interfaces as deprecated

[SamVanheer]

Some variables and functions in engine interfaces are no longer (or never were) used and should be marked deprecated using C++14 [[deprecated]] to warn against their use.

This will help to avoid accidental use of features that may not work correctly, or at all. It will also help with porting code since it immediately alerts users when their code is doing something wrong.

Symbols should be documented accordingly to explain why they are deprecated, how exactly they behave and any problems they may have.

See https://en.cppreference.com/w/cpp/language/attributes/deprecated