Diamond inheritance isn't a problem in C++ as each inheritance path is followed separately. A small price to pay to enjoy the power of multiple inheritance.
And NULL?
NULL is deprecated since C++ 11 in favor of nullptr.
C++ is old and has only accumulated, never lost.
You may want to have a look at the latest standards.
Pedantically, it's discouraged, not deprecated. NULL is a preprocessor constant defined in so many places, often in contradictory ways. There's no practical way for the C++ standards to deprecate it.
Compiler and analysis tools vendors could (and should) provide warnings when NULL is used when nullptr is more appropriate (which is always IMO).
Seeing C++ codebases using 0 for pointers instead of NULL was already a sign how up to date the respective programmers were with C++'s best practices.