Archaisms are deprecated words and phrases that are no longer in use.
Sometimes bygone language is used to emulate the writing and speech of a historical person—or to emulate a specific time and place of the past. In these cases, carefully researched archaisms could serve an artistic purpose. If you’re writing a period piece, they may be necessary.
Archaisms may also be used to allude to traditional texts. If a modern songwriter wants to evoke the ten commandments, you may hear them use “Thou shalt not”, for example.
Be careful with archaisms. If you’re going to use them at all, it’s best to have a good reason. Archaisms without purpose tend to sound overblown and pretentious to modern listeners.