That is totally false.

They have been getting replacement MiG-29s and Su-25s from allies and are starting to use f-16s from NATO nations.

"A coalition of NATO countries, primarily the Netherlands, Denmark, Norway, and Belgium, are providing F-16 fighter jets to Ukraine. The United States authorized the transfer and is providing training and spare parts, with deliveries having begun in 2024 to strengthen Ukraine's air force against Russia."

So yes, they still have an airforce. They're just getting re-supplied.

Also the Ukrainian airforce was ULTRA conservative about sorties to make sure they conserved as many fighters as possible.