Boots, they have to,
be comfortable,
resist skydrol and other Aviation fluids,
best with leather soles as synthetic will catch fire more easily if you have to run through some flames
elastic sides in case you get stuck under a rudder pedal and need to leave in a hurry, you can leave the boot
My RMs have had a simple rubber sole glued on to the leather prior to ever wearing them, just on the wearing part so the leather can still flex. (bootmaker advice) and have so far not distorted at all as the others did after "new" soles were done.
Cheers