Is the Old Testament law about being unequally yoked still applicable today?

Yes, the principle remains applicable. Scripture applies the Old Testament imagery of the yoke to believers and unbelievers, teaching that such a union will not work. Paul states by inspiration that believers must not be unequally yoked with unbelievers, and the same admonition continues today.

Les teaches that the yoke pictured two oxen joined to pull in unison, so the term implies a partnership that cannot function when one is a believer and the other an unbeliever. He notes that Deuteronomy 22:9-10 belonged to Israel's law and that Israel was an agrarian society, where agricultural practices often illustrated spiritual truths. Paul, by inspiration, said being unequally yoked will not work, and the admonition is still the same today: be not unequally yoked together with unbelievers.

Deuteronomy 22:9

Thou shalt not sow thy vineyard with divers seeds: lest the fruit of thy seed which thou hast sown, and the fruit of thy vineyard, be defiled.

Deuteronomy 22:10

Thou shalt not plow with an ox and an ass together.

2 Corinthians 6:14

Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?

Does unequally yoked only apply to marriage?

No. It does not stop with marriage; it applies whenever a believer and an unbeliever must work together for a common cause. The unequal yoke covers any such partnership, illustrated by an ox with a donkey or linen with wool. Marriage is a major example, not the only one.