So first things first:
4G is backup for 5G, we get VoLTE because
3G gets turned off no more 3G voice or data
2G aka GSM/EDGE stays around for at least a decade to cover where 4G and 5G cannot reach.
Vodafone are turning off 3G soon, and they mast share with O2, giffgaff parent company, so I can not see one turning off 3g without the other as there would be no benefit.
3G turned off would allow more 4G bandwidth, and with all of that band 40 bandwidth O2 purchased in 2018, giffgaff will be one of the most reliable networks for coverage.