Prior art I had heard of: the five teaching languages created for the 2003 book How to Design Programs. Those five languages, which are subsets of Scheme, range from the “Beginning Student” language to the “Advanced Student” language.
According to section 2.4 of Hedy’s research paper, Hedy’s main innovation compared to How to Design Programs’s teaching languages is that across language levels, Hedy’s “syntax gradually changes, rather than being extended”.
I worry that changing the language, instead of extending it, might lead to confusion when the student misremembers what level they’re working at, especially if they’d worked at a lower level for a while. That said, I really like this idea of motivating syntax, I’ll check out the paper (so far I only watched the introductory video on the site).