WhatsApp is reportedly working on a new messaging feature called message response. React to message is exactly what you're thinking of: reacting to a text message with an emoji. While it has always been possible to react to a message by quoting it and sending an emoji in response, React to Message will make the process more convenient for WhatsApp users.
Apps like Instagram, Twitter, Signal, Slack, and Facebook Messenger have long offered message reactions, so here too WhatsApp is playing catch-up with the competition.
WhatsApp's message response information was first spotted by WABetainfo, which believes the feature will be available in a future update for beta testers. Similar to Instagram DM, WhatsApp users will be able to add emoji expressions to individual messages.
There's no word on exactly what the feature will look like, as it's clearly in the early stages of development.