There’s no practical lower limit that I’m aware of. The problem is that you’re limited by the resolution of the stepper. So if you go very slow you’ll be dwelling at a single position.
In your case you’ll be taking 2 steps for every .01 mm. That means you’ll be at the same position for half a second. Not sure what you’re trying to do but you’ll need to determine if that’s a problem.
The other practical problem you may run into is that GRBL isn’t really designed to run at such slow speeds and so you may get some odd motion control and power behavior compensations at such slow speeds. There was another user on here trying to go very slowly for use on metal engraving. Two issues that I recall from memory: power was being modulated downward and I believe speed floor was higher than expected. I can’t find the forum post at the moment but worth looking for it. I believe the mitigation for the first issue was to use constant power mode. Mitigation for the second was to compile a custom firmware swapping out the motion control algorithm.
I made a POC to do this but was all hand-written g-code. This would be cumbersome to do with complex shapes without writing some code.