Form TC-40D has been eliminated because the state of Utah decided to give a personal exemption for "dependents that qualify for the child tax credit on a filer’s federal return". Disabled children qualify for that credit, so there is no longer a need for the TC-40D.
From the Utah State website:
"Federal & State Tax Reform
In 2017 Congress passed the Tax Cuts and Jobs Act, suspending federal personal exemptions and increasing the
standard deduction
for individual income tax. The federal standard deduction (line 12) is greater than previous years. Meanwhile, the 2018 Utah Legislature passed HB 2003, creating a Utah
personal exemption
for dependents that qualify for the child tax credit on a filer’s federal return."
https://incometax.utah.gov/