99 is exceedingly arbitrary. 100 is at least a nice round number, but the only justification for 99 is a misapplication of the justification for 79 vs 80. In that case, if you're treating 80-column terminals as special, 79 makes some sense as there are apparently terminals or editors (e.g. Emacs) that will wrap the 80th char in an 80-column terminal. But once you've chosen to ignore the historical legacy of 80-column terminals, then the same argument doesn't work for anything else. There's no special nature to a 100-column terminal that justifies a line length of 99.
I believe that we should have some line length limit, because code typically doesn't do great in a soft-wrapped environment, and horizontal scrolling should be avoided when possible. And 80 is definitely too short. 100 is doable, but even that sometimes feels just slightly too short. My personal preference is for 120. But 100 is a reasonable alternative.
But 99? That just doesn't make sense.
So why not use 100?
Is this a thing? I've never heard of anyone using 3-space indents until this thread, and there are multiple references to it here.