Partitive articles du, de la, de l’ and des all become de or d’ (in front of a vowel or mute h) in negative sentences using ne…pas, ne…jamais, ne…plus etc.

This rule does NOT apply to sentences using the verb être and other Verbes d’état, with which the partitive article doesn’t change.

Indefinite articles un and une become de or d’ (in front of a vowel or mute h) after a negative expression (ne…pas / ne…jamais / ne…plus … etc.) in order to express no / any.

See https://french.kwiziq.com/revision/grammar/the-partitive-article-in-negative-sentences

https://french.kwiziq.com/revision/grammar/un-and-une-become-de-or-d-in-negative-sentences-indefinite-article

and the (plenty of) examples therein.

In addition to Dimitris’ answer, beware that there are a few cases where the plural article can still be relevant in negations:

On ne prend pas de vacances → We take no vacation (we stay at home/at work)

On ne prend pas des vacances → What we are doing now is not taking vacation

Je ne mange pas de pommes → I don’t eat apples (because I don’t like them)

Je ne mange pas des pommes → I’m not eating apples (but bananas)