Yamaha used mostly blue thread lock on the TTR but that is usually found at the other end of the bolts - not near the head as in your case.
My guess is that the TTR has been exposed to salty water at some point which has caused that white corrosion in the aluminium threading.
I rarely use thread lock on my rebuilds over many years and no problems have arisen. However, I make a lot of use of the torque wrench to make sure everything is bolted up as tight as Mr Yamaha intended.
There are a few bolts that the manual says should be thread locked and I think I recall the sprag retaining bolts being included.
If you have concerns then probably best to check out the manual when re-assembling.
The following key shows that LT means to use thread lock.
This is the page showing that the sprag bolts should be thread locked.