SQL is such a shockingly good engineering choice that if you can use it I would. So even if Plate was done I would use Postgres instead of a Plate-based datastore inside my apps.

Plate serves a different purpose: schema'ing human-readable formats like iCalendar (https://en.wikipedia.org/wiki/ICalendar)[1] or JSON APIs. So even if you're using SQL within your app Plate may come in useful at the boundaries.

[1] Even if iCalendar was in JSON you couldn't write a Plate schema for it for the reasons listed here (https://github.com/seagreen/plate/blob/master/docs/prioritie...), but it serves as an example of a common human-readable data format.