Keep it short, appreciative, and neutral: give a practical reason, thank them, and offer a way to reach you. A simple line like, "Hey all — I'm cutting back on group chats to tame notifications, so I'm leaving this one for now; nothing personal, and please DM me if you need me," lands well. For work threads, try, "To stay focused during the day I'm stepping out of this chat; tag or DM me if there's something I should see." For friend groups, "Now that plans are set, I'm hopping out so my phone stays sane — text me directly if anything changes," is friendly and clear.
If you want the shortest possible version, "Muting notifications hasn't been enough, so I'm leaving to reduce noise; ping me 1:1 anytime," also works. Leave during a quiet moment so it doesn't hijack the conversation.
If the app announces departures, post your note, give it a minute, then exit so it doesn't look abrupt. When in doubt, you can mute or hide the thread instead, but actually leaving is fine if it's genuinely affecting your focus or stress. If you started the chat or play a key role, hand off ownership or say who's point of contact before you go. Most people are drowning in pings too, so a brief, courteous message plus an open door for DMs will come across as considerate, not rude.