When speaking about an expired domain’s backlink profile, this refers to the incoming links that have built up from when the domain was previously in use. Recently-expired domains that have long histories aren’t unheard of. While the pre-existing content of the domain rarely remains visible when the domain is re-registered, links on the other hand do remain, as long as the new owner doesn’t devalue or delete them.
There’s an important distinction to make here between domain flipping and website flipping. The latter mainly refers to buying and selling full websites. By full websites, I mean websites that actually have content and more often than not have traffic and revenue. When buying and selling websites, the domain name matters less as the main value there is the content, traffic, revenue, history/reputation, sustainability and growth opportunities. The same can’t be said for domain names where you’re just selling “the name” and hence it’s all that really matters.