There are Muggle-born Slytherins. While it is true that Slytherin is traditionally associated with pure-blooded wizards and witches, it is also mentioned in the Pottermore introduction that nowadays there are Slytherins with at least one Muggle parent. This suggests that half-bloods, those with one magical parent and one Muggle parent, are accepted into Slytherin house. However, it does not explicitly state that Muggle-borns, those with no magical heritage, are accepted.
It is important to note that the Pottermore introduction does not provide a comprehensive list of the types of students in Slytherin. It simply states that there are now individuals with Muggle parents in the house. This leaves room for interpretation and the possibility that Muggle-borns could also be accepted, even though they are not specifically mentioned.
It is also worth considering that the sorting process takes into account a student’s individual characteristics, values, and ambitions. While Slytherin is often associated with traits such as ambition, cunning, and resourcefulness, these qualities are not exclusive to pure-blooded individuals. Muggle-borns can possess these traits and may be sorted into Slytherin based on their individual qualities and preferences.
In the Harry Potter series, we see examples of Muggle-borns in other houses such as Hermione Granger in Gryffindor and Justin Finch-Fletchley in Hufflepuff. This suggests that Hogwarts does not discriminate against Muggle-borns when sorting students into houses. Therefore, it is possible that Muggle-borns can also be sorted into Slytherin if they possess the desired qualities associated with the house.
Ultimately, the decision of who is sorted into Slytherin is up to the sorting hat and the individual characteristics of the students. The Pottermore introduction provides some insight into the evolving nature of the Slytherin house, but it does not provide a definitive answer regarding the acceptance of Muggle-borns.