That’s the “symmetry” setting in the Sculpt Mode toolbar. Go to the Symmetry/Lock section and enable the direction you wish to mirror your strokes. Usually X is used for “left and right” side of a character for example, but this assumes that you’re building your character to be facing in the Y axis.
With official Blender, this setting is on by default when entering Sculpt Mode. Sounds like you managed to turn it off at some point.
