Google is emailing MCC owners that the UploadClickConversions request in the Google Ads API, used for offline conversion tracking, will be deprecated starting June 15, 2026.
To continue importing offline conversions, including enhanced conversions for leads, developers will need to switch to the Data Manager API instead.

This update is probably related to the Google Ads API Improves Offline Conversion Imports update published in March 2026 and explains how to handle it when using the API.
In addition to the email notification, there is also an online announcement.