The P5X is on this list so I would think the P5six would be OK as well

https://www.triathlon.org/...titions_20170711.pdf

Anyway, here is the form if you need it

https://www.triathlon.org/...nusual-bicycles.docx

Hope this helps

Edit: From the Cervelo P5 page:
https://www.cervelo.com/...riathlon/p-series/p5

...Mary Beth Ellis and Jodie Swallow’s 2015 and 2016 ITU Long Distance Triathlon World Championships...

So they were legal in 2015 and 2016, I would think they still are now.