[0xUser]

If I host my own mediawiki, will the fixed line length be enabled automatically, or do I have to configure something?

I've deployed 1.34.2, but lines still seem to span the entire screen.

[soneil]

It should be the same, uncheck Use Legacy Vector in Special:Preferences (at least, it is for me in 1.35rc3). I haven't found a setting to make that the default, however.

[bawolff]

You can make any preference default by adjust $wgDefaultUserOptions