A new feature is coming soon to the entire WhatsApp platform, which will offer users a new feature: the feature was originally called Vacation Mode, but now its name is Ignore Archived Conversations.
Currently, when you archive a conversation, WhatsApp automatically unarchives it as soon as a new message arrives from that conversation.
Thanks to the Ignore Archived Chats feature, things will soon change: when the feature is activated in WhatsApp's notification settings, archived chats will not be unarchived when new messages arrive.

When you open the new section (in development, not yet available!), WhatsApp will show you the new option:

(Source: WABetaInfo)
SOURCE: WABETAINFO