It depends. If it's using Matter-over-WiFi, yes, a firmware could change that. If it's using Matter-over-Thread (or ZigBee) it's still tied to that frequency so there's no point in changing or dropping support for the protocol except to intentionally brick the device (assuming it doesn't support another type of communication but for cost reasons most devices only support one comms frequency and that's hardwired).
Note that dropping Matter if the device has WiFi still means it needs to have some pre-existing redundancy or at least add something else instead. This also likely breaks integrations with any voice assistant in a way that would at the very least require reconfiguration, I think. It's certainly a lot less sneaky and a lot more self-destructive than just turning off a proprietary API.