Glaspell's Trifles is absolutely feminist, and was written and performed (1916) during the time of modern literature and modern drama.
But it's form is realistic and naturalistic, rather than modern.
It is rural (rather than urban, as is most modern theatre), and, as mentioned, is realistic (rather than abstract, as is most modern theatre).
The play is naturalistic in the sense that it centers on a character that is victimized by society and its gender roles.
Thus, while the play is feminist, it is realistic and naturalistic, rather than modern. Modernism is a reaction against realism and naturalism.
Just in case my answer seems a bit jumbled, let me put it into bullets for you. Trifles is:
- rural, not urban
- realistic, not abstract
- naturalistic, not abstract
- therefore, realistic and naturalistic, not modern