The delete button is removed for good reason, did its absence give you any pause to consider whether asking to remove them was advisable? If you delete those definitions then you’ve effectively broken those categories, all you needed to do was unlist them.
Your best option will be to create replacement categories and bulk migrate the posts back to them. We won’t be able to support you with things left in that state.
Why do you have 450 categories? Are you migrating an existing community with a high volume of posts?