Why some platforms show "Could not determine"
This checks each platform's own public profile page or API for a handle, but platforms differ sharply in how checkable they are from a server-side request. GitHub's public API gives a clean, reliable signal most of the time. Reddit's public profile data would be too, but Reddit has been actively blocking server-side requests to its API, so that check often comes back as "Could not determine" even for real accounts. Instagram, TikTok, and X are heavily JavaScript-rendered and increasingly block or alter responses to non-browser requests, which can make a simple fetch unreliable - rather than guess and risk telling you the wrong thing, this reports "Could not determine" whenever the signal is not clean and unambiguous.
Why this deliberately reports uncertainty instead of guessing one way or the other
A tool that always returns a confident "available" or "taken" answer, even on a platform it can't reliably check, would be wrong some meaningful fraction of the time with no way for you to tell which answers to trust - reporting "Could not determine" honestly on the platforms that actively resist checking is a deliberate choice to be useful rather than to look complete.
Why a suspended or banned account can still show as "taken" even though it looks free to use
Most platforms reserve a username indefinitely once it's been used, even after the account behind it is suspended or deleted - the handle technically exists in the platform's system even though nothing is actively posted there, which is why a result of "taken" doesn't always mean there's a currently active, visible profile using that name.
Why checking several platforms at once matters for picking a consistent handle
A handle that's free on one platform but taken on another forces an awkward choice between a consistent brand identity and actually getting your preferred name everywhere - checking every platform in one pass surfaces that tradeoff immediately, rather than discovering it one platform at a time after you've already started building a presence under a name you can't use consistently.