Shimmer Myr

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2020-11-10
  Official Ruling
  Shimmer Myr has no effect on artifact lands because they aren't cast as spells.
• WWizards of the Coast2020-11-10
  Official Ruling
  Once you announce that you're casting a spell, players can't attempt to remove Shimmer Myr from the battlefield to make that casting illegal. Removing Shimmer My after you've cast a spell won't affect that spell.

Rulings via Scryfall