Historically, Windows 10 IoT Enterprise LTSC 2021 has only been available through the OEM channel, but from 1st August, 2023 customers will be able to purchase perpetual licences without SA through the MPSA and Select Plus agreements, as well as from their CSP partners.

Find the announcement here: https://bit.ly/45nmIqV, and a note in the Product Terms here: https://bit.ly/3prduoo.