According to XDA Developers, Google Messages - the main Android messaging app - will soon notify users if they have an unread message. Additionally, unread messages will be automatically moved to the top of the users' conversations list.
At the same time, the new function will offer the following two options:
The Messages app allows users to manually set reminders for different messages and threads, and with the new feature, they will automatically be reminded.
Currently, the new feature isn’t available in the Messages beta, but it will probably be released very soon.