Subsets of JSON
Disclaimer: my project.
Matrix.org Canonical JSON
Note: This is a good spec.
JSON Canonical Form
IETF draft: draft-staykov-hu-json-canonical-form-00
Note: the mapping from JSON Canonical Form to bytes is ambiguous. It doesn't specify which characters should be escaped or which escape sequences to use.
OLPC Canonical JSON (One Laptop Per Child)
Note: not actually a subset of JSON. JSON forbids code points below %x20, but Canonical JSON (which is actually specified in bytes, not code points) allows all bytes to appear in strings.
Supersets of JSON
HOCON (Human-Optimized Config Object Notation)