A place I worked at did it by duplicating and modifying a function, then commenting out the existing one. The dev would leave their name and date each time, because they never deleted the old commented out functions of course, history is important.

They'd also copy the source tree around on burnt CDs, so good luck finding out who had the latest copy at any one point (Hint: It was always the lead dev, because they wouldn't share their code, so "merging to main" involved giving them a copy of your source tree on a burnt disk)