[MikeHolman]

Proper Tail Calls (PTC) were a problematic JS feature, and calling it "Optimization" is a huge misnomer. Only JavascriptCore (Safari) ever enabled it.

In many cases PTC causes tail calls to be slower, and it messes up stack traces. This would happen even if a developer doesn't want/need tail calls. So it was a feature that would degrade performance and debuggability of existing websites to allow new code to use a more recursive style of programming.

The main performance problem comes when you have tail calls that need to grow the stack. The engine will need to do work to adjust the stack frame.

In languages like C++, the compiler chooses to do tail calls when it improves performance, but with tail calls as a language feature, engines have no choice in the matter and had to do it even when they would significantly increase call overhead.

On top of this, there there were a number of implementation issues and corner cases. For example, most engines have some extra code layer for marshalling cross-site function calls that couldn't be removed. I also remember there being issues with Windows x64 ABI/stack frames. Tail calls from interpreter into JIT code were another implementation headache.

All this added up to have JS engines basically boycott the feature.