I was once told that if you want a stroke to print, it cannot be lower than a certain weight. I would like to know a) if this is true b) if so, does it it apply to both offset and digital printing and c) what the minimum printing stroke weight is in mm.
Yes its true.
You customarily never want a stroke weight below 0.25pts. Smaller than .25pt runs the risk of being not picked up entirely when reproduced.
The smallest line weight or detail you would be able to print depends on the line screen of the printer. But most digital presses have sub millimeter resolution. With Offset printing it's not a matter of resolution/quality and more about registration. If the line was a knockout fill (white paper showing through) there would be a chance of the ink filling in a very small area maybe half a millimeter or less. however I'm certain you would be fine if the line was a single spot color detail within the design.
There's simply no right or wrong answer here, because all printers' equipment is different. It's best to speak with the printer and they can most likely do a test for you. With offset there's a lot of factors, including ink saturation and dry time between plates. But almost anything can be done with modern printers.