Removed; Replaced with Uniques
Mythic items in Diablo 4 were announced at 2019's BlizzCon. They were the most powerful item tier. Only one Mythic item could be equipped per character, and they had the chance to roll random legendary affixes. In late 2020, Blizzard announced that it had removed Mythic items from the game [1] and replaced them with Unique items:
"Unique items will replace Mythics. We still like the idea of Mythic items, but we don't want to create an item quality that invalidates all others, so they're out for now. One of the things we loved most about them was the promise of getting random legendary powers on an item, so we folded that into our baseline legendary design."Luis Barriga · Former Diablo IV Game Director
Returning in an expansion?
One could infer from Blizzard's use of "we still like the idea of" and "they're out for now" that Mythic items could return, possibly in an expansion or patch.
TOPObsidian Heart Mythic Amulet
The only Mythic item revealed by Blizzard was The Obsidian Heart Amulet. Relative to other items, it added huge amounts of Attack and Defense, and was decked out with powerful skill-related affixes. Seeing its stats, it's not surprising that Blizzard intended for only one Mythic item to be equipped per character.

Sources
Article | Source | Date | |
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1. | Diablo IV Quarterly Update 2020 Q4 | Blizzard |
Changelog
Ver | Change | Date | |
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Alpha | Published original |