impOS (all versions) have nv support when they are on devices that have backup SRAM and support deep sleep.
imp005 does not support deep sleep at all - the silicon isn’t capable of this - and so nv doesn’t exist.
imp004m/imp006 do support deep sleep, but the silicon doesn’t have any backup SRAM, so there’s no nv. However, imp004m/imp006 also have a very low current “shallow sleep” which preserves all RAM - in fact, it just appears like your squirrel code is always running, even when the system is consuming a few tens of microamps.
In almost all cases, customers who used deep sleep on 001/002/003 projects and then moved to 004/006 just use shallow sleep.