There are several third-party plugins and converters available that can help you convert an InDesign file to an older version. Some popular options include:
If you are working within the Creative Cloud ecosystem, Adobe tries to bridge this gap automatically using its cloud servers, provided both users have active subscriptions. How it works: how to convert indesign file to older version
IDML is universally compatible with InDesign CS4 through the latest Creative Cloud (CC) releases. how to convert indesign file to older version
Here’s a concise, practical guide to convert an Adobe InDesign file to an older version. how to convert indesign file to older version