/sbin/nologin is set as the shell, if user with that shell logs in, they'll get a polite message saying 'This account is currently not available.' This message can be changed with the file
/bin/false is just a binary that immediately exits, returning false, when it's called, so when someone who has
false as shell logs in, they're immediately logged out when
false exits. Setting the shell to
/bin/true has the same effect of not allowing someone to log in but
false is probably used as a convention over
true since it's much better at conveying the concept that person doesn't have a shell.
nologin's man page, it says it was created in 4.4 BSD (early 1990s) so it came long after
false was created. The use of
false as a shell is probably just a convention carried over from the early days of UNIX.
nologin is the more user-friendly option, with a customizable message given to the user trying to log in, so you would theoretically want to use that; but both
false will have the same end result of someone not having a shell and not being able to ssh in.