WhatsApp has introduced an option called Keep in Chat, which allows users to prevent messages from vanishing in a disappearing message thread. You can long press on a message so it stays in the thread. However, the person who sent the message will be notified, and they'll ultimately decide whether to keep it in the chat. If not, the message will vanish into the ether as usual when the timer runs out.
A bookmark icon will appear next to saved messages. You can access all the ones you've saved in a Kept Messages folder, where they're organized by chat. WhatsApp says it's rolling out the feature gradually and everyone should have access within a few weeks.
On the surface, it seems like Keep in Chat undercuts the entire point of disappearing messages. However, WhatsApp suggested in a blog post that there may be times when you need to save a message (such as a voice note or someone's address) that includes important details. In any case, those who sent the message will have the option to veto saved messages and perhaps protect their privacy.