Is cutting of hair forbidden for a sikh in sri guru granth sahib??
I mean for a sikh, not an Amritdhari Sikh.
Best answer for this is to look at the History of Sikhism and role of Sehajdhari's or slow adopter. The slow adopters were people who were usually defined as Sikhs who believed in Bani i.e. believed in every aspect apart from keeping the 5 k's (including Kesh) i.e. did not take tobacco, or alcohol, read bani and did the paat's, got married in Gurudwara etc.
One of the failures of modern day Sikhism has been not to recognise so called "Mona" Sikhs, hence why people drift to cults liek Radaswmi etc.