Every definition of a “backdoor” in computing implicitly or explicitly considers it hidden/covert.
In the house analogy you don’t see the backdoor when approaching the front. If it was just “an alternative everyone knows about and can be broken easier than the front door” then it probably would have been called “a window”.
Most login forms have a weaker option like a SMS 2FA or password reset fallback. Nobody calls it a backdoor. It’s just a crappy second front door, or window.
I'm probably mistaken, but I've always referred to password resets as backdoors. Is there another term they could be classfied as?
The Free Software Foundation (FSF) calls the update system used in Windows 10 a "back door" [1], I think because it installs updates automatically. This sounds like nonsense to me, because it implies that I installed a back door on my own machine by enabling automatic upgrades (on Trisquel).
It's meaningful that the Windows 10 install method has no (official) way to disable it, but I don't think making something optional could make it not a back door, if it was one before.
Even when automatic updates are disabled, I'm not going to be reading every update so the effect seems mostly the same, regardless of whether updates are automatic or not.
The FSF's definition of "back door" (at the bottom of the linked page) is "any feature of a program that enables someone who is not supposed to be in control of the computer where it is installed to send it commands" which leaves a lot of ambiguity with the words "supposed to be". I am not sure how to interpret this definition.
[1] https://www.gnu.org/proprietary/proprietary-back-doors.html#...