iPhone (Messages)
Settings → Messages → turn off “Send Read Receipts”
Android (Google Messages)
Open Google Messages → tap your profile icon → Messages settings → turn off “Read receipts”
Android (Samsung Messages)
Open Samsung Messages → More options → Settings → turn off “Send read receipts”
Settings → Privacy → turn off “Read receipts”
Facebook Messenger
Messenger → profile icon → Privacy → turn off “Read receipts”
Instagram Direct
Instagram → Settings and privacy → Messages and story replies → turn off “Read receipts”
Telegram
Settings → Privacy and Security → turn off “Read in private chats” (if available)
Microsoft Teams
Settings → Notifications → disable “Read receipts” (wording varies by version)
Slack
Preferences → Advanced → disable “Read receipts” (if available)