The behavior was switched so that translations can now be toggled off per-post with the translation icon near the date
Part of the reason for the removal of the whole-topic toggle is that some people were confused about its purpose when they clicked it without a translated post in view (“nothing happens”), and there were additional complaints about its always-visible prominence.
I think we can reconsider the who topic toggle if it’s more useful, but we should reconsider the design and maybe add more feedback about what the toggle does. Reddit’s toggle is clearer because it always shows supporting text explaining what it does.