As long as the void has excellent cross-ventilation (no 'blind alleys') and neither the solum (the base of the void) nor the walls are really wet you should be OK (though beware that underfloor insulation frequently disobeys the laws of Building Physics!). I would also suggest a breathable membrane at the bottom, sealed at all joints and perimeters, to prevent 'windwash'.

Same question I raise everytime ... the floor has been a means of vapour removal from the house for many years. If that huge dehumidification area is made vapour tight, what is the replacement mechanism for vapour removal?

I use a vapour permeable membrane to support the insulation, as it is vapour open, and good tear resistance. There are certain temperature swing weather conditions when condensation can occur on the under face of the insulation, and the VPM helps mitigate this. Fix to the joists with felt tacks (staples as temporary fixing till in place). Use a magnetic tack hammer - so much easier eg https://www.ebay.co.uk/itm/134780987552

Trad upholsterers often carry the tacks in their mouths and just offer up the hammer each time. Potentially fraught with danger for the numpty.