
Say something short and matter of fact before you mute or leave. I usually send a heads up that I am cutting back on notifications and will mute the chat for a while. I add that I am not upset and just trying to focus. Ask them to tag you or DM if they need you and say you will respond. If it is an event or planning thread, add that you will pop back in when dates are set or ask someone to text you for decisions. If you truly need to exit, post the same note, leave the group, and send a quick DM to the person who started it so there is no ambiguity.
On iPhone Messages, open the group, tap the name at the top, toggle Hide Alerts, and if everyone is on iMessage you can tap Leave this Conversation. On Android Messages, open the chat, tap the three dots, go to Details or Group details, then Notifications and set it to Silent or turn off notifications. In WhatsApp, open the group, tap the name, choose Mute notifications and pick Always or the time you want, you can also Archive to keep it out of the main list, and if you leave a group now only admins are notified. In Messenger, open the group, tap the title or the info button, choose Mute until you turn it back on or Leave group. On iPhone you can also go to Settings, then Focus, set Work or Do Not Disturb, choose Allowed People and schedule it so only key contacts break through. On Android go to Settings, then Notifications, then Do Not Disturb, allow starred contacts and set a schedule so group chats stay quiet by default.