Free dining is a package discount, which is generally used instead of room rate discounts or other bundled items. The types and amounts of discounts vary as occupancy in the resorts do - they want those rooms full, and when they're not, they offer deals to get them filled up.
When the dining plans start returning, free dining offers will likely also show up, but again, the types of offer and the availability (dates and limited quantity) of them will depend on what resorts are seeing booking shortages.
As with any package discount, do compare the offer against other offers available at the same time to see which one actually works best for your vacation.