Didn't think it was a big deal since most of everyone knew of Dragon King's return cause the spoilers were posted on reddit when the leaks happened and this isn't someone dying or anything. Just a cool hype moment everyone wanted to happen. I don't see any future credit pages having GT spoilers since I myself am not that much of a fan of the 3rd series in Index and think it's overall disappointing. This was just a fun surprise at best in terms of excitement so you won't need to worry about any future spoilers on the credit pages.
Sorry if I negatively impacted the novel reading experience for you. Index fans mostly didn't seem like the type that care about spoilers. Won't happen again cause I'm not that interested in GT anyway.
Something was smeared out on the credits page....
TBH I was expecting Touma to just reach up and destroy Curtana when she tried to kill him when he was laying down.