Hello @Rakesh Daphale
Welcome to Microsoft QnA!
There is no Official Documentation
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"To my knowledge, classic pipelines are not officially deprecated, it has not been announced. That been said, I doubt a MSFT employee can provide you an estimated date when it is going to be deprecated. Usually, when Microsoft announce a feature as deprecated the feature is still supported for a good number of years," from @Alberto Morillo
Also this : https://devblogs.microsoft.com/devops/disable-creation-of-classic-pipelines/ mentions that they are not going away
I hope this helps!
Kindly mark the answer as Accepted and Upvote in case it helped! Regards