Spinster was a term used to describe femcels from the 19th century onward. The main cause of spinsterhood wasn't the usual factors behind inceldom but rather all the wars which killed a large bulk of men in the pre-modern and post-modern periods. During the 19th century, slutty behavior was frowned upon and women looked to men for provisioning due to the scarcity of office jobs; so rather than become sluts, women who could not find a suitable man to become a husband might remain sexually abstinent.
In the 21st century, "spinster" is essentially a term for a confirmed femcel. Some women who are exceedingly cautious about getting into a sexual relationship, or tired of failed relationships with badboys and unable to find a man who is marriage material, and yet not single moms either, may become femcels.