I was not aware of any attempts to create a single language that would be "simple Spanish". Still, you can find a lot of books written in simplified Spanish. They are called "lecturas graduadas" and are adapted for students of different levels.
The Common European Framework of Reference for Languages (CEFR) defines six levels of knowing a language:
- basic speaker (A1 and A2);
- independent speaker (B1 and B2);
- proficient speaker (C1 and C2).
In Europe, Plan curricular del Instituto Cervantes is the standard reference for Spanish. For each level, teachers decide which vocabulary and grammatical structures students already know and which are appropriate for this level, thus roughly defining a 'simplified' language that can be used to write textbook exercises or "lecturas graduadas".
However, this simplicity is not strict: we have no standard "word lists", and teaching materials intentionally include less frequent words.
According to the approach of Instituto Cervantes, one of the essential skills that a student has to learn is
aplicar procedimientos habituales en el uso de una lengua extranjera, como deducir del contexto el significado de alguna palabra o alguna estructura gramatical que no conoces.
to apply standard techniques when using a foreign language: for example, to infer from the context the meaning of an unknown word or grammatical structure
-- El Cronómetro. Manual de preparación del DELE. Nivel B1. Edinumen, 2011.
There are even special exercises designed to train the skill of guessing the meaning from the context. In comparison to this approach, a controlled simple language where all the words are already known would be too artificial.