Hair holds a deep significance for many demographic groups, often along racial lines due to differences in style and texture. This frequently involves hair length. For some people, hair has religious significance, for others it is more an expression of heritage, but opressors have forced people to cut/change their hair as a means of stripping people's cultural expression for a long time. Shaving newly enslaved black people as a means of erasing their cultural heritage goes back to the 15th century, as many groups had distinctive styles and slave owners wanted to impose conformity. Forcing Indigenous Americans to cut their hair was done to homogenize children removed from their peoples and punish/demoralize adult men, stripping both of them of an important religious and cultural signifier in the process.
A lot of modern hair discrimination has its roots in this more explicit racism, denouncing hair that isn't in line with western-european beauty standards as unprofessional, unkempt, or unsightly. Length of hair and specific styles hold value to many different ethnic groups today, just as it did hundreds of years ago. Many black people see the display of black hairstyles (including long braids, dreads, afros, etc.) as a form of cultural reclamation, many indigenous americans still view hair length as religiously meaningful, tons of Sikhs, Muslims and Jews have strict beliefs regarding hair/beard cutting, the list goes on. Forcing these people to conform or face discipline is absolutely discrimination, and these groups are often a different ethnicity or race than the person mandating the hair be cut.
Is forcing people to maintain a certain hair length always solely racist? No. It can be discriminatory in a plethora of ways. It can also be sexist, queerphobic, and/or a form of religious discrimination. I was subject to the purely sexist aspect of this by old white guys for having long hair as a white, cis-het teenage boy, no racism involved. The label for any discrimination relies as much on who is being discriminated against and how it is applied as it does the views of the person enforcing it, making it an intersectional issue
A good rhetorical example of this multitargeted discrimination would be the banning of necklaces with stars on them. Is it inherrently discriminatory on its own? Not in a vacuum, no one is born wearing a necklace with a star. But consider two major religions that involve star iconography (judaism, islam) and you can see how this rule is antisemitic and islamiphobic whithout ever mentioning jewish or muslim people explicitly. Which form of discrimination it is contextually depends on the person experiencing it. Hair is no different. Making a black guy cut his dreads/braids is both racist and sexist when viewed in this light, as it targets a cultural symbol (a black hair style) and is likely unevenly applied across genders (black girls aren't usually required to have short hair). I hope this answers your question, if asked sincerely, and here are a few sources if anybody wants to learn more:
1991 Duke Law piece on the intersectionality of hair, race, and gender, with the key takeaway quoted below.
"Judgments about aesthetics do not exist apart from judgments about the social, political, and economic order of a society. They are an essential part of that order. Aesthetic values determine who and what is valued, beautiful, and entitled to control. Thus established, the structure of society at other levels also is justified."