I see that I am not the only one to have complained about this--there are lots of posts about categories disappearing, in the forums and elsewhere.
Then it's better to go here:
https://todosupport.microsoft.com/support?product_id=todo&auth=1&nf=1&fromAR=1
Follow the prompts to report your problems. This is for tech team to be aware of the problem and fix it. Please describe as detailed as possible, so that the problem will be solved faster.
If there is no way to recover the list, this a major crisis for me. (Why wasn't I instructed to back up my lists if that needed to be done? Why isn't it automatic?)
Different users have different needs. Few users need to export their lists, and Microsoft wants to make the app appear in minimalist style. I also understand that you want To Do to automatically export your list, because it is more convenient. But there are also many users who prefer to decide whether to export or not by themselves, and some users even think that the default automatic export is abuse of privacy. If you have this need, go to:
Microsoft To Do · Community
and make a upvote about this issue. This is to let Microsoft developer team hear your ideas and improve the products and services.
I totally understand your disappointment, and I know you just want to make Microsoft better. I can't guarantee that the problems you encounter will be resolved very soon, but your feedback matters a lot.
Wish you all the best!