I can think of 3 benefits of these valves:
1. For carb’d engines to prevent back-firing: Carb’d engines, in the inlet manifold is wet with fuel. Hard deceleration (e.g. from 4000 rpm in 2nd gear, snapping the throttle shut) causes extremely low manifold pressure (= very high vacuum) that scours this wetness thru the engine, too wet to ignite until it reaches the exhausts. The gulp valve limits the vacuum to prevent such scouring. I won’t have carb’s so won’t need gulp/decel valves for this reason.
2. For all engines, to reduce oil leaks caused by hard deceleration (e.g. same 4000 rpm in 2nd gear scenario). Hard deceleration generates the most blow-by gases, e.g. this chart (posted by Arisides on Jag-lovers:
https://forums.jag-lovers.com/uploads/s ... rHMJm1.pdf)

Jaguar vents the crankcase via the throttle plates which are closed when decelerating; instead, the decel valves open to provide alternative relief of crankcase pressure, thereby preventing leaks. Some mod’s introduce PCV valves which would directly provide some relief however there is a monstrous size disparity, a piddly little PCV would be irrelevant. Everyone needs deceleration valves for this reason? Yet still Jaguar deleted them. I guess because they weren’t making sports cars anymore?
3. For V12’s with Laycock de Normanville overdrives; to protect the overdrive. LdN OD’s incorporate a relatively tiny clutch to prevent free-wheeling when decelerating. Decel valves would dramatically reduce the torque required to be transferred via this clutch when hard decelerating (from 4000 rpm in 2nd). I do have an LdN OD and I would like it to last …