There WAS a mapcyclefile cvar in 1.18.0.3, and there still IS one as of the 9/14 update. It is hidden, meaning you have to use sm_cvar to access or set its value.
Valve may be trying to discourage its use to avoid "confusion" -- confusion they caused by going to this mapgroup system and still having a mapcyclefile cvar that parts of the dedicated server still uses. But it IS still there and it IS still used. Sourcemod still uses it too, unless you configure your maplists.cfg to not use mapcyclefile for anything.
Now, regarding that nextlevel cvar --
It doesn't make a lot of sense to manipulate that cvar if doing so is going to force a map change at the end of the round, rather than the end of the map, as the cvar description states. Even if it doesn't change the map at round end as the description states, deciding WHEN it would be appropriate to set nextlevel could be rather complicated. What if some other plugin changes it? All of this leads to more code and maintenance of that code, or cases you can't handle, or at least not without more code and time put into it.
WHY BOTHER WHEN IT IS A LOT SIMPLER TO JUST REPLACE THE CANNED MESSAGE???
Since what SourceMod thinks the next map is supposed to be is what you're going to get for the next map anyway, all this plugin needs to do is intercept the CS:GO server's canned message and replace it with one that says what SM thinks the next map is supposed to be.
I usually run
cvarlist after an update too.
Just keep in mind, that doesn't always show you everything that's really there (and really USED)...