Upvote:2
This forum post from 2011 says that the user contacted a Thai Airways representative in Hong Kong and was told that rackets should be checked-in since they exceed the allowed cabin baggage dimensions:
TENNIS RACQUETS:
"Due tennis racquet is over-sized than the allowance of cabin baggage, it is required to checked as a checkin baggage."
This does put you at risk of having to check-in the racket, since as you said
Squash rackets have maximum dimensions of 686 mm (27.0 in) long and 215 mm (8.5 in) wide
and the maximum allowed dimensions for carry-on luggage are:
maximum length 56 cm (22 inches), width 45 cm (18 inches), thickness 25 cm (10 inches).
To be sure, try tweeting Thai Airways asking them specifically about this problem. You might get an updated answer this way
As an alternative, the racket might just fit diagonally in a flat rectangular 56Γ45 cm box or carry on luggage, though -- and certainly would fit in the long diagonal of a 56Γ45Γ25 luggage. So you can try this solution.