Explanation is not clear and we don't have any idea about the use of the Migration API to simplify the process as mentioned in this post.
We need more explanation about Bookmarks and how we can do migration. You need considering when writing this kind of post that developers that will use this documentation are not working in the internal team of prismic to know all these details.
Hope we get more explanation about how things can be done.
PS : I discover this depracation by using Migration API and receiving this error when calling https://migration.prismic.io API : The custom type does not support UID
Thanks for reaching out. We announced this via emailed to all our users mid-December, but it's always helpful to hear feedback on how we could communicate better and improve the documentation related to those big changes.
What do you need more details on, is it the transition between the two? You would just need to add the uid field to any custom type that you were querying using bookmarks, and use the current bookmark as the uid field. The Migration API can be used to simplify the process but this is the logic.
Then once this is done, you would need to update your queries to retrieve documents via the UID rather than the bookmark.
Let me know if I can give you more precise detail. Feel free to share your set up and what you're working with/trying to do, so we can get really specific
