In MC 1.6.4 there used to be this option:
I:repairScalingModifier=5
B:repairCostScaling=false

Now it's missing and every repair adds less to the tool durability, rendering it useless after using it for a long time.
Could this function be added back, please?

Vanilla TCon already does that, this just allowed to modify the rate of it. If you want actual limited repairs, use the config for that.

Exactly. And I'd like to disable the diminished repairs like I was able to before.
Because that function is now missing, I can now repair tools only until the diminishing return reaches critical value when it's simply not worth it or even possible to repair the tool.
That function is now missing and repairs are esentially not unlimited.

..unless you set a maximum amount of times it can be repaired, as i said above.