It's not uncommon to have to retype a WhatsApp message you were typing when you exited it to do something else. Unlike Instagram, where you don't lose the message you're typing if you exit the app, WhatsApp requires users to retype the content from the beginning. However, according to WABetaInfo, WhatsApp is testing a feature to save message drafts in the latest beta for iOS 24.18.10.72. However, this feature is available on both WhatsApp Web and WhatsApp Desktop (Windows, Mac).
As soon as you are in the middle of typing an unfinished message in a WhatsApp chat and exit the chat or app completely, the message will automatically become an unsent message or draft. So once you revisit the WhatsApp app, the conversation will automatically be labeled as "Draft" at the top with the message you were typing intact.

Many have commented that the WhatsApp draft save feature is very similar to Instagram’s draft save feature that has been around for quite some time. The core mechanism remains the same and as WABetaInfo notes, “With the introduction of the new draft label, users can quickly identify which chats contain unsent messages without having to open each chat individually.”
However, you should note that this draft feature currently only applies to the device where you saved the draft and does not appear when accessing WhatsApp via the desktop application, and is not cross-synchronized on WhatsApp servers.