Shudra was not untouchable , shudra in varna hierarchy was a social group which was supposed to serve other 3 viz brahman ,kshatriya and vis . untouchables were those who were out of pale of caste system they were also known as other names such as panchamas , malecchas . that is why there is a constitutional term 'scheduled castes' for them which is a legal term to integrate them in larger hindu society i.e caste . because there was no caste name for untouchables/panchamas , constitution gave them caste name and it appeared in a schedule of constitution hence the name S.C.
Also you can go by elimination
Are jats brahmins ? NO
Are jats kshattriyas ? Well some people might have a misconception . majority of jats have been agriculturist and not the mercenaries like kshatriyas . There are instances jats having common names with rajputs e.g. Tomar . there are some isolated instances of a group capturing political power and employing brahmins to latch them to rajput lines of solar or lunar dynasties . they invented genealogies to gain legitimacy as only the kshatriya was allowed to become administrators according to hindu tradition .
it should be kept in mind that while the elite who captured power gained social mobility his kin groups remained at the same status .So jats are not kshatriyas .
Are jats vishya ? no . hereditary occupation of jats was not trade and commerce .
Are jats untouchables/panchamas ? No . untouchability was worst form of social exclusion , never faced by jats .
Are jats shudras ? you decide