This is a pretty popular pattern known as Entity-Attribute-Value [0]. It's used by many products where a) data model needs to be very flexible and allow new attributes without schema changes, or b) a typical entity has a large number of possible attributes that may or may not be set for all entities ("sparse" attributes). WordPress uses this to store post metadata, Magento uses this to store product attributes and most of other data, Drupal uses a variation of this to store all the posts and other content you create… I have too much experience with this model to be surprised.

[0]: https://en.wikipedia.org/wiki/Entity%E2%80%93attribute%E2%80...