Time-travel debugging is something different – time-travel debugging means you can actually step backwards through the execution to try and see how you ended up with the bug.
Merely being able to inspect the state of (local) variables further up the stack frame is a much more limited proposition, even if it can still be useful.
yet very often wished I had it (for […] javascript […] mostly)
Both Firefox's and Chrome/Edge's devtools allow you to do that, don't they? Click on an entry in the stack frame and it takes to the corresponding code line and shows you the state of the variables relevant at that point.
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Time-travel debugging is something different – time-travel debugging means you can actually step backwards through the execution to try and see how you ended up with the bug.
Merely being able to inspect the state of (local) variables further up the stack frame is a much more limited proposition, even if it can still be useful.
Both Firefox's and Chrome/Edge's devtools allow you to do that, don't they? Click on an entry in the stack frame and it takes to the corresponding code line and shows you the state of the variables relevant at that point.