Beets — Food safety facts
Safe to share · Cats · full breakdown on the linked food page
Cooked plain beets in small cubes are safe. Oxalates mean very large amounts could irritate the kidneys, so keep it a treat.
Short answer
Yes, a small piece of plain cooked beet is safe for cats. The oxalate content only becomes a concern with large amounts, so a tiny treat is fine. Offer it plain and watch for any vomiting or diarrhea. If anything seems off, give your regular vet a call.