I'm Miles from the Social Care team at Shopify. When you purchase a domain, it generally has a one-year expiry. If you don't renew it, then it does become available for purchase again. If you believe your domain was renewed in time - or that it was purchased less than 12 months ago (April 2020 or later), then we'll need to take a look for you.
It looks like we’ll need to access your account in order to resolve this issue. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback. Please visit https://bit.ly/3cJkx8V and log in to your account to create a support request.
Since this is the next step I've marked this reply as the solution for the benefit of others who may discover this topic. Thanks!