If the app posts a "X left the chat" banner, send a one-liner first, then leave a minute later. Keep it about your notifications and bandwidth, not the people or the content. Something like: "Hey all, I'm trimming chats to reduce pings, so I'm stepping out of this thread—DM me anytime if you need me." You don't owe a long explanation or apology; a single thanks or a heart emoji is enough. If the app doesn't notify anyone when you exit, you can just leave quietly or, even easier, mute the thread and check it on your terms.
With family, soften and offer another channel: "Love you all, the constant pings are a bit much for me, so I'm going to mute/leave this chat—text me directly and I'll respond." That reassures them it's not rejection and prevents someone from taking it personally. With friend groups or event-specific threads, a short wrap-up works: "I'm not keeping up here and don't have much to add—catch me 1:1," then bounce. If you're an organizer or key participant, hand off anything open before you go; otherwise, no need to overthink it.