No, deaths didn't stop and the infant population was very different.
Letby faced charges on 17 babies. She was found guilty of harming only 3 who would have been cared for at a level 1 unit.
Two of the babies she is accused of murdering would have been there, out of seven. The other was an attempted murder case. There is no official definition or register of unattempted collapses, so no comparison we can make there
So, Lucy is accused of killing two babies who were in level 1 care in a 14 month period. The unit was then downgraded and she stopped working there. Four babies in level 1 care died as early or late neonatal deaths in the next 14 months.
Letby was accused of causing about half the infant deaths on the unit. So it looks as if outcomes for level 1 babies did not change at all when she was removed from the ward. Still, there would be plenty of ways to explain a change that didn't involve murder.
Edit: this was a reply to @ClockwiseHoneysuckle
You persist in ignoring the stark contrast with the death rate in the two preceding years, and the fact that the majority of the babies concerned would still have been at the Countess of Chester if the unit had been a Level 1 unit. You cannot get away from the fact that there was a sudden fall in deaths and collapses of Level 1 babies when Letby was finally stopped from working.