Indexes, no, they can be recreated by restic repair index.
Snapshots, maybe, because they are what makes up your “points in time” for the backups - they are not your actual data, but you need them to be able to restore a consistent set of files from the points in time that you backed up your data. But if this is a concern, the question becomes, why would your snapshots files be lost and not at the same time the other files? It’s more relevant to ponder the risks of corruption on disk, and if that happens it’s usually something that affects a few snapshots, not all of them.
Keys, yes, as they cannot be replaced without at least one key working to access the repository.
The config file, yes, as it contains information used by restic to process the repository correctly and even if you can work around a missing config file, it’s clearly something you would prefer not having to do 
I hope I got all of that right. Someone please correct me if I wrote something that is wrong.