There is a very small handful of secret areas and interactive objects. For example, you can actually use the outhouse in Korean Folk Town. Nothing to the extent of other games, however.
There are numerous explicit and implicit references to pop culture, including a straight-out Thanks, Obama meme in FriendStory and spoof characters of Bowser and Anna in Mushroom Kingdom. The characters in Kerning Tower are all named after gemstones and the entirety of Omega Sector is a spoof of Power Rangers.
All character classes have unique dialog in certain storylines. Specifically: Black Heaven and Tenebris. For example, in the Labyrinth of Suffering, the voice telling your character not to give up is different for each character:
Resistance characters have Vita
Explorers have Sugar
And so on
Full Metal Alchemist: A long time ago, there was a set of equips in-game modeled after Edward and Alphonse
Naruto: As above, there was a set of equips modeled after Team 7 (or at least Naruto and Sakura)
Bleach(?): There is a set of equips in the game files from a long time ago modeled after Renji and Rukia. I can't actually recall if this made it to GMS, however.
Mabinogi: A small event where you could obtain a Nao chair. Mabinogi likewise has Maplestory events
Sword Art Online: The first official crossover event, featuring the SAO dungeon and equips modeled after Kirito, Asuna, Heathcliff, etc.
Attack on Titan: Several minigames and the battle against the Colossal Titan, as well as outfits/chairs.
Re:Zero: A set of daily quests, outfits modeled after characters, and medals
Card Captor Sakura: Daily quests, chairs, outfits, and medals
Evangelion: Dailies, chairs, outfits, minigames
JMS has had the following events:
SAO, AOT, Re:Zero, CCS, Evangelion as mentioned above
Vocaloid: A set of rhythm minigames, chairs, and outfits modeled after the various Vocaloids
Pop Team Epic: Outfits and chairs modeled after Pipimi and Popuko
Nexon has not confirmed Spanish farming to be safe or legal, at least in no post or comment I can find. The only sources I can find suggesting this have been a handful of Reddit posts claiming "a streamer did it" and that a GM in live chat said it wasn't explicitly against the rules (no evidence or screenshots of this claim).
Bumbertyr also tested the hack detection system with a Spanish farming Kanna, and did not trip the system, but this only confirms that the hack detection system active in December 2018 was not detecting or banning for Spanish farming.
Nexon has not explicitly spoken out against the technique either, but from another player's perspective it looks very much like hacking, and placing Kishin in the exact same spot when it expires looks a lot like macroing.
Ultimately the safest way to play the game is to actually play the game normally. I cannot guarantee that Nexon will not change their minds and decide Spanish farming is against the rules, nor can I guarantee you won't be reported by other players.
You'll have to be more specific about what's happening.
For starters, the quest "The Secret Square" is only available for Explorer thieves and Dual Blades, not Dawn Warriors.
There are no quests for Cygnus Knights which lead you into the Secret Plaza (the Resistance hideout map). There are a handful of quests where you interact with Edelstein citizens and the Resistance, but none of them involve entering the hideout itself.
But more importantly, you'd have to specify what you mean by "has changed" and "not approachable". What is happening? Can you not enter the Secret Plaza using the sewer pipe on the left-hand side of Edelstein (in between the police station and the school)? Is it related to this issue?
