We can mark all messages from existing conversations or chats on the OnePlus smartphone as read without having to open each individual conversation.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Messages app
Step 2:  Open the  menu
Step 3:  Tap on  Mark all as read
  1. Open the Messages app
  2. Open the menu
  3. Tap on Mark all as read

OnePlus Instructions