I believe in the UK British Standard AU159f covers tyre repairs, including motorcycles. As Kildare Man says, other places will have their own regs. There's no suggestion that m/c tyres cannot be repaired and I'm pretty sure most tyre manufacturers will say the same thing.
However, in the name of balance, the general recommendation (or standard) seems to be that external repairs (liquid, plugs or string) are seen as temporary fixes and the only long term fix should be a tyre-off inspection and repair from the inside. Different numbers of repairs can be done on the same tyre depending on speed ratings etc.
Me? I've generally taken mine to be repaired at a bike shop, largely because I'm a cack-handed oaf. I've happily run tyres from nearly new (because that's when you always seem to pick up punctures) for over 10k with a couple of repairs without issue. Would I trust an external plug or string? Probably. If I made a temporary repair on the go would I ever get round to arranging a 'permanent' repair. Highly doubtful. Would I bin a good repairable tyre because it had a small tack in it. No chance.