I wore the Mk10 Immersion Suit on every flight from it's introduction.
In 1984 I was ordered by the Sqn boss to stop wearing and return it to Flying Clothing.
I insisted that (according to Air Clues and the RAF Combat Survival Org) I was entitled to dress for the worst survival conditions I would encounter - ie, January, at night, over the north sea.
He said recent policy over-rode those requirements; the Nimrod was now classified a Category AA aircraft because of it's in-built redundantcy - 4 engines, multi hydraulics and high survivabilty rate - I argued but was told to wind my neck in !
All current aircrew at ISK at that time (circa 566) had a Mk10 hanging up in the locker room - but only about 20 faithfulls wore them - OC Eng did a survey and went to Cmnd about the wearing/servicing schedules for these garments and proved his flying clothing staff could not provide 100% checks for 100% of the stock in the required time scales, when only about 13% were actually being worn
So they were replaced by "Quick-Dons" and this was the real reason for my being denied my "body armour" - not enough staff - too difficult!
The parachute thing is similar - cat AA - no need - good chance of survival - as demonstrated by the lovely Art Stacey AFC in the Moray Firth