It’s called subject inversion.

It is usually understood as a syntactic rule which applies after the deep structure of the sentence is set, to produce its surface structure. You may want to look at this topic about inversion part IV. A. Relative pronouns.

In the deep structure, as you know, the order is SVO, and it stays as is in most sentences. The footnotes of the page state :

Optional inversion – Generally speaking, use inversion for formality, avoid it for familiarity.

Regarding your particular example, the following sentence would be a simple way of defining the same thing :

Les chevaliers portaient autrefois un bouclier, appelé écu.
Or :
Le bouclier que portaient autrefois les chevaliers s’appelle un écu. (or est un écu.)
Un écu est un bouclier que portaient autrefois les chevaliers.

Hence the definition you found.

Inside a relative clause introduced by que, if the subject is not a pronoun, it’s possible (but not required) to place it after the verb:

Comment savoir ce que Marie a pensé de son intervention ?

Comment savoir ce qu’a pensé Marie de son intervention ?

It’s also possible when the clause includes an indirect object. Especially in the case of indirect object pronouns, these are valid word orders:

Je n’ai pas compris ce que Marie nous a dit.

Je n’ai pas compris ce que nous a dit Marie.

Adverbs (like autrefois) are free to move so your original sentence could be written either way:

Bouclier que les chevaliers portaient autrefois.

Bouclier que portaient autrefois les chevaliers.

Note that if the subject is a pronoun the inversion is clearly wrong:

Je voudrais savoir ce que tu penses.

*Je voudrais savoir ce que penses tu.