Disappearing online is the easy part - just stop using the platforms. I've done it so many times I can't even count and my mutuals pretty much expect it from me at this point. I'm actually in the middle of one of my "disappearing phases" right now, but this one has lasted much longer than previous ones and I don't think it'll end any time soon. My advice is to delete the apps if you find yourself constantly feeling the urge to check them and try as hard as possible to fight the feeling that you need to reinstall. Not even for a second or "just to check something." If it doesn't help, consider making an alt account for certain platforms that nobody you know knows about and only use the platform through that alt. Reframe how you look at the platforms. Something that's been keeping me off this time around is the sentiment of my brain being fed garbage by techbro billionaires that couldn't care less about me. You don't have to deactivate if you don't want to, in fact, people will probably notice your disappearance sooner if you do. Take advantage of the platforms that allow you to alter your status. I still have Discord and Steam, but my appearance has been set to offline for years now and my mutuals know that it is, so it's not alarming when I suddenly become inactive on those platforms for an extended period of time. Don't forget to set your status for platforms that feed through, e.g. my Steam status might show offline, however, launching Overwatch through Steam will still give me an online status for Overwatch and may even alert people in game when I start playing.
EDIT: blocking everyone and leaving servers does work, but I don't really recommend it. It always left me questioning if people noticed or if they've said anything about me, which I think inadvertently fed my urge to keep up with the social media platforms when I was supposed to be gone. As weird as it sounds, at least with the inactive/appearing offline route, you still have the ability to check in every so often. I also found it humiliating to need to be reinvited to a server that I "disappeared" from.