(Topic titles are clickable links)
Jan 2020 thread to tell people to get ready to switch eventually as the first small groups of users were migrated. This became a very active discussion thread on the changes.
June 24 official thread with more details:
Announcement | Changes to our Legacy SmartThings Platform
August 2020 thread with official timeline for when features will be removed:
Updates to the SmartThings Platform
November 2020 thread discussing the final sunsetting of the classic app and (thanks to @Automated_House ) listing which countries have access to which official features, such as the lock manager and smart lighting.
And the following two threads were not official announcements, but are where staff have been tracking reports on the new voice assistant skills:
New SmartThings Alexa skill (2020)
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Updates to Google integration
Other user-created threads on the migration, including community FAQs, can be found at the following tag:
https://community.smartthings.com/tag/migration2020
If you are looking for information and discussion on the new platform from a coder‘s point of view, see the developers section of the forum. Just be aware that as of this writing this is still very much a work in progress.