For some of them I kind of lost access so I would need to write emails to instagram or whatever, for others I would need to remember and to access to each one of them and delete them.
The hardest would be gmail with people having my contacts from like 10 years ago…
I would recommend just hiding/removing your content on social media then let it go inactive. There may be some reason in the future you need to utilize the service (looking at you events that are only organized on facebook and restaurants that only post their hours on instagram)
Instagram - posts can still be checked in a browser without logging in. Imginn exists too if you want to browse a profile.
Facebook - Anecdotal, but I have not had a Facebook account in 20 years and I can safely say I’ve never once had FOMO or trouble finding information on an event. If it’s a private social gathering that’s worth going to, someone will invite me.