The double-click speed setting returns, and now lives on System Settings’ General Behavior page. Before you ask why it’s not on the mouse Page, it’s because it affects touchpads too and that has its own page, and duplicating the setting on both pages seemed messy and ugly
As long as the value is synced, I can't imagine how it would be bad to have in both pages; especially since that's where a user would expect to see it.
There's nothing wrong with multiple ways to find a common setting