Thanks for the referenced thread!
Indeed we already have numpy.sin and scipy.special.sindg; adding scipy.special.sinpi would look “crowded”. It would have been nice if there were no scipy.special.sindg function 
I checked the spec of IEEE tanPi; it does agree with sinPi/cosPi in terms of signed zero and infinity. So yes, adding sinPi and cosPi would be minimally sufficient.
In fact, sinPi and cosPi are already defined in IEEE 754-2008, while tanPi only made its way into IEEE 754-2019. Apparently the tanPi function had some technical difficulty for standardization; see https://grouper.ieee.org/groups/msc/ANSI_IEEE-Std-754-2019/background/tanpi.txt for a discussion. But the linked post doesn’t explain why and how tanPi got into IEEE 754-2019.