deducting vacant investment land sold 10 years ago

no. too late to amend prior returns.

No, but maybe partially kind of.

You must report the loss in the year you sold the property.  That's absolute.   You should have reported the loss 10 years ago (2014?).  That created a capital loss that might offset capital gains from other investments, and you could deduct $3000 over your other capital losses.  Any loss on the property that was not deducted in the year you sold the property would be carried forward to the next year.  This process repeats until the loss is used up.  

While it is true that you can't amend your return to claim an additional refund more than 3 years after the deadline, you can still file an amended return.  What you would need to do is prepare an amended return for the year you sold the property, include the loss, use it to offset any other capital gains, and calculate if you had a carry forward loss.  Then, prepare an amended return for the next year to report the carry forward loss, offset any gain, deduct the extra $3000, and carry forward the remaining balance of the loss.  Repeat that for every year after.  If you get up to 2021, 2022 and 2023, and you still have a loss carry forward, then you could amend those years to deduct the $3000 and carry the loss forward again, and claim an additional refund based on the $3000 deduction.

But you can't just file an amended 2021 return showing the carry forward loss, you would have to file all the previous returns so the carry forward loss was properly documented.  I believe this is allowable (even though you won't get any refunds from the amended returns more than 3 years old), but you might want to speak to a tax professional.  You would also have to spend considerable time (and maybe money) refiguring your last 10 years of tax returns to see if you even have a loss that carries forward the whole 10 years so there is anything left to use now.  Obviously if the loss was less than $30,000, there is no point because it would be used up by now, even if you had no other capital gains.