Is foot a plural possessive noun?

No, the noun 'foot' is a singular noun, a Word for one body part or the base of something.

The plural form is feet, a Word for two or more of these body parts or bases of things.
The plural possessive form is feet's.
A possessive noun indicates that something in the sentence belongs to that noun.
A noun that does not end with an s forms its possessive by adding an apostrophe s ('s) to the end of the Word.
A plural noun that does end with s forms its possessive by just adding an apostrophe (') to the end of the Word (arms' or legs').

Example sentences:

He placed his boots at the foot of the stairs. (singular)

My foot's injury kept me out of work. (singular possessive)

She had her feet massaged and her toenails painted. (plural)

The table left its feet's impressions in the carpet. (plural possessive)