As someone who had lifelong near-total absence of smell and then, surgically, gained a full working sense of smell, I can report that after about twelve years my brain typically no longer reports "chemical poison" or [panic abort] when I eat food anymore. About a year ago I had a jug of milk spoil and I smelled it and it smelled like spoiled milk and I actually cheered and texted all of my friends, because it was the first time I'd ever been able to detect that the milk had turned before drinking it.

As this article notes, Covid may require the neural linkages for smell to start over from scratch, and I can attest that in my case when they started building from scratch, flavors were extremely strange for at least the first year or two. Rosemary smelled like windex, cilantro smelled like lawnmower exhaust, cold scrambled eggs smelled like a campfire.

The story has patients at the 0.5 year mark freaking out about how it hasn't recovered yet, but it took me years simply to build enough connections to be able to eat without confusion. For the first decade, I essentially had to treat eating as if I were training a neural net: Look at my food, study my food, consider my food; Lean forward and smell my food, lean back and consider what I just experienced. For some food I had to repeat this a couple times to understand what was going on.

Nowadays I do this casually or not at all with foods I understand, but any time I encounter a new dish, I make a point of doing this, because otherwise I might take a bite of rosemary eggs and my body will dump adrenaline because it thinks there's a chemical fire. I wish I could reach these people and their doctors and the Covid treatment community somehow and say —

"You need to treat this like learning a new language from scratch. Write down a list of spices in your home and then, every single day, pick one jar and take the time to just sit there and study it with your eyes and focus on it with your mind and smell it, set it down, think for a minute, and then do it again. It will be confusing. It will make no sense. It may smell of nothing, it may smell incorrect. That's okay. This will take time."

It took about ten years before I could so reliably smell the smell of hot oil in a pan that I don't have to set a timer for cooking, because I don't have any memories of that smell to regrow connections to — I had to develop that experience from scratch, as if I were a child. Those who've lost their sense of smell at least have memories to work from, that can be focused on and used to help the brain direct the rebuilding process. It's critical to make the effort even if you can't smell anything at first.

(I would start with cinnamon or pine needles, personally, since those are the most meaningful smells of my childhood — both were able to punch through the damaged nose enough to register something. This varies wildly by culture — "mom's curry powder" or "a thimble of your favorite scotch whisky" are just as effective if you have the strong memories to connect with.)

EDIT: Book recommendation to support the retraining experience: "Spice", by Stuart Farrimond. I found this book invaluable at learning what I was perceiving and how to begin making use of spices this year — for example, one day I came up with lavender salt and smoked paprika on slices of fresh-cut pear — using methodologies that do not require being able to smell food to comprehend and understand. I still have no clear grasp of what thyme smells like to me — I haven't been able to affix it in my memory yet! — but I can look it up and consider how it links to other spices and start to use it anyways.