As to splitting functions each to a file, never adopted that because I am the author and it was my choice. Now this was all C code, not C++ so with each language we followed the rules that worked for us.
In other words, standards are great since we have so many to choose from. For instance, indentation was one I told them to take a hike on. That rule would change with every manager. It wasn't worth it to change the code base to meet the new standards. If they wanted it changed, we'd ask for additional hours (money) which shut down the request quickly.