It will be written off as a capital loss on Form 8949. As a capital loss, it will offset capital gains. If you don’t have $43k in capital gains to offset, then up to $3k can be used to reduce your ordinary income. Any remaining loss will be carried over to next year, where it will again be used to offset capital gains and up to $3k in ordinary income. Lather, rinse, repeat until the losses are used up.
I would point out that unless you have total income that exceeds the 0% capital gains bracket ($96,700 + your standard deduction for MFJ), there isn’t really a tax benefit to offsetting long-term capital gains because they would have been taxed at 0% anyway. So, this year, you may want to sell enough stocks to bring your total income up to where you will have $43k more in capital gains than the 0% capital gains bracket to maximize the tax benefit of the loss. You can use this strategy to re-set the cost basis on investments that you intend to continue to hold for more than a year.
AJ