Well, we've had a few since visiting the plane of Kamigawa, which is clearly based on a number of Japanese mythologies
Liliana of the Veil has maintained a good value since its release thanks to its power in formats like Commander, making the SDCC version all the more valuable among players.
There are so many directions you can go with these, it really fuels my creativity
Like most trends that boomed during the pandemic, some of this activity has declined
They offer everything from simple stat boosts to game-warping abilities
Until then, we may see the mechanic dusted off and reused in a Masters set of some sort, where enfranchised players with more experience are likely to engage and leverage the strange timing and rules associated with cumulative upkeep