Users don't notice a deprecation warning. But they might notice adding a "time.sleep(10)" immediately at the top of the function. And that gives them one last grace period to change out their software before it breaks-breaks.
In addition: if CI is the only place the issue shows up, and never in a user interaction... Why does that software exist in the first place? In that context, the slowdown may be serving as a useful signal to the project to drop the entire dependency.
ETA: To be clear, I don't do this as a substitute for a regular deprecation cycle (clear documentation, clear language-supported warnings / annotations, clear timeline to deprecate); I do it in addition before the final yank that actually breaks end-users.