This is controversial, but a couple days ago I broke up with a girl after 6 months, I noticed the majority of people highly suggest blocking them on all social media and deleting their number.
In my opinion, this shows that you care. So much so, that you couldn't stand seeing their face on their timeline that you HAD to block them on everything just to get them out of your head.
Girls are attracted to INDIFFERENCE and fed off of your emotional reaction. By doing this, you are showing that you care, you are showing weakness. She WANTS you to block her on everything and delete her number. That gives her validation that you truly cared about her. Psh.
You show higher value if you just don't care, no matter how beta you were in the relationship. She will go crazy if you don't try to contact her either (liking IG Pics, watching stories, obvious texting her) yet you have the access to do so. This especially if you want to get back with them or plate them in the future (I do not suggest this, rather, find other women)
This especially if you were the one who got dumped, as she sees you're just living your happy live unaffected by her toxic ways, it will cut down her ego. Sure delete pics and throw out her shit. But unless you truly can't get over them, there's no need. Why? You have an abundance mindset. Things didn't work out? Okay, on to the next one.
Also, if they are a former plate, why delete her number? If you ignore them, the boomerang effect will take place and she will come right back to you. If she texts you and you say, "who's this?" she knows you were butthurt about how things ended and lose attraction because she knows she got to you.
Of course, it's a different story if they did something terrible that you had no choice to cut them out of your life. But otherwise, be indifferent, be unaffected.