Are Shimano quick links reusable

Dunno, but i re-use them untill i feel they become easier to click together then i replace. 
- i also havnt broken a chain.

Officially, Shimano says they aren't re-usable. Unofficially, they are definitely re-usable. If it feels loose or sloppy get a new one. Otherwise don't sweat it.

Unpopular opinion but they're designed for single use. 

You've most likely invested training hours, travel costs/time etc into this race, don't let 10$ quick link spoil the fun.

Good luck racing!

Agree w/ this for race use, for every-day riding I re-use them and haven't had any issues in many years of and miles of riding.

The thing with using new quick link or even new chain for race run is, you never know if it will fail or not as you haven't tried it before. There is plenty of wc downhillers that could tell you how much nicer it feels to snap brand new chain out of the start gate than used one. Not much better right? Another thing is, new chains are slow unless properly prepped (this is one of the most stupid things wc mechanics do for ages and it really grinds my gears, sorry I had to vent my frustration lol). But I digress, I agree with first three or so guys in this thread, yes, officially they are not meant to be reused, but if you connect or disconnect the quick link carefully and it feels nice and tight when reconnecting it, at least you know it isn't defective because you have already ridden it before the race.

Taken my chain on and off a bunch, didn't even realise they were 'single use'.

Not Shimano, but Sram Powerlock (11 and 12spd), I always reuse them (for example to clean the chain and the like), never had a problem in the last 8 years of doing that. And I'm sure there are cases where I reused a single quicklink over 10 times.