Looking for a word describing "habits which lost their original purpose"

Solution 1:

The practice itself is called a shibboleth (in one of its senses; 1c.2 below):

shibboleth (ˈʃɪbəˌlɛθ) n

1. a belief, principle, or practice which is commonly adhered to but which is thought by some people to be inappropriate or out of date
2. a custom, phrase, or use of language that acts as a test of belonging to, or as a stumbling block to becoming a member of, a particular social class, profession, etc

[C14: from Hebrew, literally: ear of grain; the word is used in the Old Testament by the Gileadites as a test word for the Ephraimites, who could not pronounce the sound sh]

Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003

I wouldn't recommend that you verb it.

Solution 2:

You might be thinking of vestigial, which refers most often to organs or body parts found in organisms which no longer need them, as "vestigial tails" in developing human embryos, or "vestigial eyes" in blind cave fish. It can be extended to any function which was once useful, but no longer is.

Another, unlikely, possibility, is hangover (not the alcoholic variety), referring to conditions which persist after the original requirement has passed: "This function is a hangover from a previous design, and can safely be eliminated."