It is all very individual and dependent upon where you were before your keto journey began. If one is a very metabolically unhealthy person, it might take him or her years to heal, years before they can handle starchy or higher glycemic carbs, if ever. If the person is metabolically healthy or an athlete doing keto for optimal health or performance, that person may not need to worry about carbs at all. She may even want a bit more carbs to target around work outs or sporting events.
You just have to figure out what is going to work for you. Anything can be keto if eaten in the correct quantity. But if it is something sugary or starchy, would you be able to put on the brakes in less than 20-30 grams of carbs? Some people find that even the smallest amount of a sugary carb, even fruit, will set them off into terrible cravings. Some people find that while some fruits are fine, if they have a potato, the appetite brakes come off. I have no problem eating fruit and vegetables in moderation, but put a tortilla chip in front of me and I will keep going until the bowl is empty. I am not capable of eating less than 20-30 grams of carbs where tortilla chips are concerned.
The question is always: why would you want to go back to what caused your health problems in the first place? I may stop keto some day and go towards a more paleo type diet, but I know I will not go back to how I ate pre-keto.
I will add one last thought: sometimes we don’t know how bad we fell until we feel better. It’s possible that oatmeal or something else you used to eat contributed to your inflammation, but your inflammation was enough that you really didn’t notice any difference when you consumed the oatmeal. Once your inflammation decreases and then you eat something that doesn’t agree with you, you will notice because you now know what if feels like to feel good. I hope that makes sense. I am mildly rambling here.
I did a carnivore trial for a while. I am no longer carnivore, but I stick pretty close to it. I know pretty quickly if something I add back in causes trouble because my body and/or joints will hurt. I don’t think I would have found these intolerances without my trial. I guess my entire message is boiling down to: you will need to experiment with yourself and see how and when you can reintroduce foods, which ones, how much, and possibly scale back if necessary.