There’s nothing in either the metric system or the English system that sets limits on things.
Take the bolts and nuts issue brought up above. In the English system, the pitch is specified in “threads per inch”. There are standards, but that doesn’t preclude anyone from making something off standard. In fact, there are places where an adjustment screw has used an ultra fine thread that isn’t part of the standard (as in UNC and UNF).
The same for metric - but in this case they specify the pitch as the distance between threads. There are standard thread pitches, but there is nothing that prevents someone from using an unusual pitch.