Some languages (Python, Tcl, R and possibly others) expose the stack as part of the language/standard-library, and thus an implementation that DOES do TCE canno

Some languages (Python, Tcl, R and possibly others) expose the stack as part of the language/standard-library, and thus an implementation that DOES do TCE cannot be compatible. In python, it's mostly visible through exception objects. In Tcl and R, it's in common use because it is standard practice to do things in your caller's scope instead of your own. In C++ (and any language with destructors), many routines are invisibly not TCEable because the language semantics require the destructor to run after the tail call returns - which means that a change to a different part of the code determines if a specific function is TCEable or not. So, despite your claim being technically true, it is in practice often informally specified by the language or standard library.