It's unclear what the reason for first introducing the original hack here (for [tabindex="-1"]:focus {...}) was. Seems something that may have been useful/necessary in SuitCSS, but don't think BS ever relied on this. https://github.com/twbs/bootstrap/issues/18330
It's since been modified to only apply when the browser wouldn't apply a visible outline anyway based on its own heuristics (the :not(:focus-visible) part) https://github.com/twbs/bootstrap/pull/28437/
But now, thinking this through more...in browsers that do support this pseudo-selector, what this is essentially saying is redundant: don't apply outline in cases where a tabindex="-1" element receives focus but the browser wouldn't normally apply focus outline". at best, this is unnecessary. at worst, this actually overrides things an author may explicitly be trying to do with adding :focus { outline: ... } explicitly.