" Form 8990...... is supported in TurboTax Business, supporting Form 1041 returns."

You are correct and the form is simply mislabeled in the 1041 module (see screenshot - you have to click "Limitation on Business Interest Expense" in the list of forms).

It is labeled correctly in the 1065 and 1120-S modules, however.

"Form 8990 is still not available. Any update on when it will be available?"

The form will most likely not be available, the probably reason being the language in the instructions (see screenshot).

Link: https://www.irs.gov/pub/irs-pdf/i8990.pdf