The pile of high quality regression testing Google search has is miles above any other product I have worked on; but the number of possibly generated search result pages and the number of supported browsers conspire to make the pile of possible breakages even higher.
Even so, I felt better about removing code in that JavaScript codebase than just about any other I have contributed to, and frequently did. (My total line count in JavaScript was negative for a while).
The problem was much less about the difficulty of it and more that it wasn't difficult or high enough impact for the amount of time you needed to spend to detect these cases and prove their safety. The low hanging fruit of stuff that already passed all the regression tests was often already scooped up and the piles of cruft all had one edge case with some test or were hard to trigger and confirm manually that it was fixed.
Even so, I felt better about removing code in that JavaScript codebase than just about any other I have contributed to, and frequently did. (My total line count in JavaScript was negative for a while).
The problem was much less about the difficulty of it and more that it wasn't difficult or high enough impact for the amount of time you needed to spend to detect these cases and prove their safety. The low hanging fruit of stuff that already passed all the regression tests was often already scooped up and the piles of cruft all had one edge case with some test or were hard to trigger and confirm manually that it was fixed.