A tax would be better than a ban. The tax can increase every year. And the money from the tax could be redirected to help people transition to electric.
The downside of a tax is it's harder to implement, so there will be inefficiency in the system. A ban is easy.
The long term effect of such legislation, especially if replicated in other jurisdictions, is that OEMs will cease to manufacture LNG/LP appliances because of lack of demand.
If OEMs no longer sell LNG/LP appliances, vendors that make LNG/LP components (burners, orifices, combustion chambers, etc.) stop making them. Over time, appliances become unserviceable.
Then consumers must purchase electric appliances, even if they are not a viable option—because they are the only option.
You are correct; there is no cause for immediate concern for existing appliances and existing residences. But over time, only electric appliances (and parts) will be available for purchase, and that is precisely the intent.
Just as California emissions laws generally set the standard for automobiles that are available for purchase in the US, this legislation will have far reaching affects across the country.
The downside of a tax is it's harder to implement, so there will be inefficiency in the system. A ban is easy.