You could call "broken" a "participle adjective". Certainly "broken" is derived from the verb "to break".
Note that a participle adjective is simply a category or sub-class of adjectives, they function in exactly the same way as all other adjectives, to modify nouns. I do not think that the distinction is very important. English very freely derives nouns from verbs, verbs from nouns, and adjectives from both.
Your dictionary says:
The participial adjectives are a major subclass of adjectives. They can be distinguished by their endings, usually either -ed or -ing. Some exceptions to the rules include misunderstood and unknown, which also function like these special adjectives. They are called participial adjectives because they have the same endings as verb participles.
These adjectives are really meant to function like any other adjective: basically, they help to describe a noun. They might come from a verb form, or they might merely imitate the structure, but they always function as a descriptive adjective
An article on ThoutCo says:
Comparative and superlative forms of participial adjectives are formed with more and most and with less and least--not with the endings -er and -est.
This last difference (in how to form the comparative and superlative) seems the only reason to even use the term, "participial adjective" or "verbal adjective"; otherwise an adjective is an adjective, whatever its form or source.
I would also note that "broken heart" is a set phrase, perhaps even an idiom. It is a metaphor, referring to the heart as the supposed seat of emotion, and a person with a broken heart is one devastated by grief or sadness, particularly for a lost love, but also from any other sad event. The term frequently occurs in popular song and verse, and has been in use for a very long time.