Don't do it. Here's some reasons why.
When my current partner got Facebook quite a while into our relationship, I felt I needed to do the same and go back and delete some stuff. Wasn't his request, just something I felt like doing, don't remember why. But I went and deleted about 4 albums (which I hugely regretted) and the big life events (In a Relationship, Anniversary etc) which I don't regret much. But the pictures I do - there was one album in particular which was from when one of my nephews was born. There were lots and lots of photos of me, my ex and my new-born nephew and I just felt like it was a slight against my current partner at the time as he has an amazing bond with my nephew. The result? I don't have any pictures now of the first few weeks of my nephews life and my brother was pissed as they lost all their pictures when the battery exploded in their laptop, and now they don't have very many pictures of him as a new-born, either.
The truth is, you can delete moments from your life from social media but you and everyone will not be forgetting them any time soon so what difference does it make? The actual reality is that now that I am in such a good place in my life, and so is my ex, it's entertaining, nostalgic and just feels nice to sometimes look back and be reminded of happy, or funny moments in history, even if we were together for it, and especially if they were seeded in times that were shit, and the last year of my relationship with my ex was miserable for both of us before we eventually called it quits. It just reaffirms the strides and changes I've made in my life up until now, too.
Just this week Facebook reminded me of a post I made about 6/7 years ago. Me and my ex had fallen down some concrete steps together walking out of a wedding reception (a little drunk obviously) and I ended up with a fractured elbow and he had a little cut on the back of his head glued. It was hilarious then, and even funnier now looking back, for both of us.
If you were completely and utterly miserable with your ex, and looking back only brings you pain, then start deleting. But if you look back and some things make you laugh and smile, then don't bother. Those moments are important to look back on!