Trending @Capgemini; If I resigned due to manager behavior, will our service letter be affected?

No but ideally please report the behavior so it doesn't happen with anyone else

same thing happening with me... mental harrasment and trauma... it's from upto the dept head...

Please don't report anything to anyone, it's not that company that will hear your problems, they will take your complaint otherwise and put all the allegations against you only. Just resign, serve notice period, leave and write anonymous reviews for others like I am doing.