Read it yourself

For studies involving humans categorized by race, ethnicity, national or social origin, sex, gender identity, sexual orientation, religion, political or other beliefs, age, disease, (dis)ability, socio-economic status, or other socially constructed or socially relevant groupings, authors should:

• Explicitly describe their methods of categorizing human populations

• Define categories in as much detail as the study protocol allows

• Justify their choices of definitions and categories, including for example whether any rules of categorization were required by their funding agency

• Explain whether (and if so, how) they controlled for confounding variables in their analyses

It certainly doesn't say don't do such studies.