Do Squibs have to be pure-blood?

Question

If a Muggle was born of a wizard/witch and a Muggle, is that person a Squib, or is that reserved to people born from pure-blood families?

(Related: Is a Squib a Muggle?)

Answer 1

Squibs aren’t only pure-blood - they can have a Muggle parent.

If a wizard and Muggle have a nonmagical child, it’s a Squib. In the Pottermore writing on Dolores Umbridge, it’s mentioned that her parents (a wizard father and Muggle mother) have two children together - Dolores, a witch, and her brother, who was a Squib.

Dolores Jane Umbridge was the eldest child and only daughter of Orford Umbridge, a wizard, and Ellen Cracknell, a Muggle, who also had a Squib son.
_{^{- Dolores Umbridge (Pottermore)}}

This shows that Squibs can be born to a wizard/Muggle couple - in that case the nonmagical child would still be considered a Squib, not a Muggle, despite having one Muggle parent. It also shows by extension that Squibs don’t have to be pure-blood, as any child with a Muggle parent isn’t.

Answer 2

No I don't think so. Kendra Dumbledore was definitely a Muggle-born witch:

'Dumbledore's mother was a terrifying woman, simply terrifying. Muggle-born, though I heard she pretended otherwise -'

'She never pretended anything of the sort! Kendra was a fine woman,' whispered Doge miserably, but Auntie Muriel ignored him.

And yet nobody has any problem speculating that Arianna was a Squib:

'- proud and very domineering, the sort of witch who would have been mortified to produce a Squib -'

Harry Potter and the Deathly Hallows - p.129 - Bloomsbury - Chapter 8, The Wedding