This will negatively affect Tesla, but probably less than the share price drop implies. because this tax relief change will only affect customers in the US.
Tesla sells to the world, and the rest of the world are increasing these subsidies and issuing mandates that will force people to switch to EVs. So there is no shortage of buyers for Teslas and other EVs globally. If the domestic market drops off they'll have to export more, but they should still be able to shift their stock.
A more direct impact of this change will be to discourage other domestic US manufacturers from building EVs. This will be unfortunate, because their international competitors are moving rapidly toward EVs in light of the forementioned mandates, so there is a very real chance that US manufacturers will find themselves behind the curve in years to come.