iceythe gave you the most concise answer and essentially accomplishes what you want without the need for any add-ons.
The Edit Mode > Mesh > Symmetrize feature appears to be what you want, but it has some limitations. You must be in edit mode for it to apply symmetry, and you can only specify negative & positive only after you have applied the symmetry. In addition, you can not specify whether you want the symmetry to be non destructive as a modifier.
Just for fun, I made my own “auto mirror” add-on. I called it Neltulz - Symmetry.
It does appear to be very similar to the “Auto Mirror” add-on, but that was not intentional. It seems, the desire for a quick and dirty auto mirror feature is very high in Blender, so there exist many ways to do the same thing, quickly and easily.
I recommend checking out my mirror add-on: (☠ Retired) Neltulz - Symmetry 1.0.1 - Blender 2.8
Pros:
- Works in both Object Mode & Edit Mode.
- Slice, Cut or Mirror…
- ability to set the cut location & rotations to “local” or “global”.
- It uses the mirror modifier, but then it immediately applies it, unless you have chosen to keep the modifier.
- As a bonus, you can add the Neltulz - Symmetry panel to any custom pie menu, or context menu for extremely fast access.
Cons:
- No threshold options yet
- Does not support radial symmetry
- Does not allow you to symmetrize only the selected vertices,edges,faces. It symmetrizes ALL or nothing.
