Until the user-based authorisation system of the server puts you in place.

Well, if it were at a gap, as would be expected of a normal staircase, it would have enough space for the door to open and for someone to enter it from the side.

I would not want to live in that room either way.

Yes, but it is also good to know why it is recommended.

Yeah, that kind of a condition would require the maintainer to patch the source of the non-updated program.
And that would be fine if there is just a little change, with an alternate function available but if the change requires changing the logic of the application, you are essentially expecting the package maintainer to do the software developer’s work.

The deprecation process is a good way to prevent this.

No, pacman -S firefox will not update your firefox.

pacman -Sy firefox will update your firefox and nothing else.

If you have done pacman -Sy once, then your list of packages and their versions gets updated.
From then on, using pacman -S <package> on any package, whether or not it was already installed, will now get the new version of it.

On the other hand, if you have not updated for long, then if you run pacman -Su to update, it will update nothing, because it looks at the old package list and compares it to installed packages and all of them match.
If you were to use pacman -Sy and then pacman -Su, then it would do the update, similar to pacman -Syu.
If you did pacman -Sy yesterday and then do pacman -Su today, then it will update up to yesterday’s packages and will ignore any updates from that point to today.

This can be considered analogous to apt update and apt upgrade.
If you run apt upgrade without apt update, you only upgrade upto the packages that you got until the last apt update.

If arch used apt, then in this case, the recommendation would be for never use apt update without using apt upgrade right after it.