Reversed part of this change for the next release (4.0.21), because it has had unintentional consequences, when certain links have been automatically linked and there has been no toggle to prevent it; most notably this occurred in templates, which prevented some templates from parsing correctly or at all, but the issue is possible in other situations as well, which is why the box existed in the first place.
The default state of the checkbox will still be checked for new wiki pages, with the checked state preserved on the next edit. However, the checkbox now appears, where in previous versions it had been hidden. The checkbox is still hidden for wiki comments, because most of the consequences are not relevant there; the linking behavior can be suppressed with PLAIN tags without causing additional problems.