Green Sun's Zenith

• WWizards of the Coast2011-06-01
  Official Ruling
  If this spell doesn't resolve, none of its effects occur. In particular, it will go to the graveyard rather than to its owner's library.
• WWizards of the Coast2016-06-08
  Official Ruling
  In most cases, if you own Green Sun's Zenith and cast it, you'll shuffle your library twice. In practice, shuffling once is sufficient, but effects that care about you shuffling your library (like Psychogenic Probe, for example) will see that you've shuffled twice.
• WWizards of the Coast2016-06-08
  Official Ruling
  If Green Sun's Zenith is countered, none of its effects will happen. Notably, it will be put into its owner's graveyard rather than shuffled into its owner's library.
• WWizards of the Coast2016-06-08
  Official Ruling
  If you own Green Sun's Zenith, but an opponent casts it (due to Knowledge Pool's effect, for example), that opponent searches their library for an appropriate creature card, then shuffles that library. That opponent then shuffles Green Sun's Zenith into your library. You won't shuffle any library in this case.

Rulings via Scryfall

• WO
  Wizards of the Coast2015-02-03

  Green Sun's Zenith was banned in modern on February 3, 2015. Green Sun's Zenith was removed mostly for diversity's sake among green decks. Source: https://magic.wizards.com/en/news/arcana/brief-history-modern-banned-list-2015-02-03
• WO
  Wizards of the Coast2024-12-16

  Green Sun's Zenith was unbanned in modern on December 16, 2024. Banned during a time where green decks were too strong, the card offered a level of consistency and flexibility that was difficult to combat, often narrowing the range of cards you'd consider playing. To help increase metagame diversity, Green Sun's Zenith is unbanned in Modern. Source: https://magic.wizards.com/en/news/announcements/banned-and-restricted-december-16-2024
• WO
  Wizards of the Coast2011-09-20

  Green Sun's Zenith was banned in modern on September 20, 2011. On turn one, this can give the acceleration of a Llanowar Elves by getting a Dryad Arbor. However, this ends up with fewer different decks being played in practice, as Green Sun's Zenith is such a good choice that there are fewer green decks that do anything else. The DCI hopes that banning Green Sun's Zenith increases diversity among Modern green decks. Source: https://magic.wizards.com/en/news/feature/explanation-september-2011-br-changes-2011-09-20-0