https://github.com/nostr-protocol/nips/blob/master/12.md
Interestingly, it is noted that the tag name (#t) is case sensitive, but nothing is noted about the tag value. 🤷🏻
Discussion
Amethyst is creating the event by adding the original and lowercased tag values:
"tags":[["t","OpSec"],["t","opsec"],["t","OSINT"],["t","osint"]
Also the value for r tags needs to be case sensitive simply because URLs are case sensitive, but it would be nice to have t tag values case insensitive. It would probably remove some of the ambiguity between implementations.