To constrain one bone to another in Spine, you can use a transform constraint. First, select the bone you want to constrain, then click New... and choose Transform Constraint. This will allow you to enter the target mode where you can select the target bone by clicking on it. Ensure that the target bone is not a descendant of the constrained bone. You can adjust the constraint settings to control how the constrained bone behaves in relation to the target bone, such as limiting movement to specific axes or adjusting the mix to achieve the desired effect. If you want the constrained bone to move in the opposite direction, you may need to adjust the offset or mix settings in the transform constraint properties.

For more detailed guidance, you can refer to the Transform Constraints - Spine User Guide.

Create a new transform constraint, as before. In the tree properties for the constraint, click the blue "nubs" for translate X and Y (or drag them straight across, but clicking is easier). Also choose whether you want to map from local or world (left Local checkbox) to local or world (right Local checkbox). Usually you'll check or uncheck them both.

Now the target bone moves exactly with the source bone. If you want the target to move more or less, you can change the translate mix, just like Spine < 4.3. If you set the mix to negative, it will move the opposite direction.

Transform constraint has more features now, but just doing the above gives you the < 4.3 functionality. It's a straight across mapping from X -> X and Y -> Y, mapping from 0-100 to 0-100 so you get the same movement.

For example, if you mapped from 0-100 to 0-50 then it would move half the speed. The mappings are especially interesting with Clamp, though it can be tricky to understand.

Note there are two Match buttons now. The one you are used to in Spine < 4.3 is in the Offset section. It works like before, except you no longer need to set the mix to 0 before you match -- it always does the match functionality as if mix was 0.