This feature request was discussed with Campbell Barton during meeting 2021-02-23
It is hard to select the corresponding vertex on the flipped axis side when the topology is non symmetrical and the corresponding vertices on the flipped axis side is slightly offsetted.
In order to fix this, it would be great with a "proximity slider" that selects vertices on the axis flip side that is within the range of the corresponding coordinate of your current selection.
UI mockup for viewport options
UI mockup for operator "Select mirror"
Use cases below:
Current behavior works great when Topology mirror == True and the mesh on both side of the axis is connected (and the topology was "built symmetrical"). Even if vertices on the axis flip side is slightly offseted.
Current behavior does not work when the topology is built symmetrical, but the topology on both side of the axis is not connected (like the eyes in this case). And the mesh is slightly offsetted on the axis flip side.
Current behavior does not work when the topology is not completely symmetrical and the vertices on the axis flip side is slightly offsetted.