They have depreciated, just not as much, there's perhaps a 5% difference between a Niro ICE and EV from 2020 looking at cars of similar age and mileage far as I can tell, when you look at actual list excluding PiCG discount.
Depreciation is low though and can't see it changing for a few years, so wouldn't even consider a GAP on something cheap like the Niro EV we have on order whereas on my PHEVs I expect more of a drop as the government chase those as the next planet killer after they have finished with diesel bashing and they become the next untouchable.
No they haven't depreciated, they have gained price. In 2021 a top of the range e-Niro was £39k, the £40k one I listed above is 10 months old and has alsmot 8,000 miles on it. Yet is about £1k more expensive than when it was new. The cheaper BEV E-Niro I listed would have cost £31k (with grant) yet is £2k more expensive for a current used model.