https://lists.blender.org/pipermail/bf-blender-cvs/2018-November/116518.html
In short, the mirror modifier can now act as a sort of bisect tool on top of the usual mirror operation (as the video shows). It might be useful for creating some types of meshes procedurally.
The contributor is Benjamin Sauder.
thats pretty nifty and very useful!
Awesome!!!
Thanks on the heads up.
very nice. Another dream come true.
The naming is not ideal. It does not give a result close to what bisect tool provides.
These options are transforming Mirror modifier into a Symmetrize modifier.
To be consistent with edit mode, option should be called Symmetrize.
When I saw title of thread, I really thought that was a bisect modifier and was confused because there would not be reason to merge both.
Bisect tool produces a cut or eliminates geometry on one or both side of section plane.
But mirror modifier adds geometry on other side of symmetry plane.
Wow, very good, I instantly had to download a new build and try it out.
Unfortunately, it doesn’t do the bisect if the mirror checkbox is turned off. I had hoped we could finally have a normal bisect modifier without mirror.
–> would it be possible to change the modifier so that it can also only bisect without mirroring? And then maybe change the name to “mirror / bisect” or something similar.
I was literally just now wondering how cool it would be for a feature like this to exist
Finally, I’ve been waiting for this feature far too long. 
One of those things I missed from 3dsmax, great news!
Nice
It would be awesome if they improve other modifires aswell for example the build modifire.
just wow on this modifier. Absolute fire.
Oh wow. 
Dat rotate function!
Thx for this !
Can i ask again for radial symmetry and to add modifiers on collections ?
Great!!!
thank you!