I've often found it interesting that as the push for TDD has come to the industry, the rule that I was always taught to enforce backwards compatibility in your code seems to have fallen out of fashion.

For example, when Golang makes guarantees that new versions of the language won't break code working on old versions of the language that's backwards compatibility. These days, that has become a revolutionary feature while I always considered it to just be an expectation.

Because I make a point to maintain backwards compatibility, I tend to see very little benefit from TDD. It slows me down significantly. If I'm working on an MVC monolith however, eventually NOT having that test suite gets really scary.

Working with smaller pieces and enforcing backwards compatibility vs working with huge code bases that need a huge test suite is more desirable IMHO.