Estōte occurs five times in Lingua Latina Per Se Illustrata by Hans Henning Ørberg. This textbook, written in the 20th century, teaches elementary conversational classical Latin by example. The story is set in about 150 A.D., is limited to basic language, and often presents simplified versions of excerpts from classical works, sometimes quoting directly when the language is basic enough. It's my understanding that Ørberg took historical accuracy very seriously in LLPSI. I take Ørberg's use of estōte to mean that it's basic Latin that everyone should feel comfortable with.
Here are three examples.
p. 209, illustrating gerundium:
Industriī estōte in scrībendō, discipulī!
p. 282, illustrating the plural imperātīvus futūrī:
Magister: "Posthāc bonī discipulī estōte, puerī! Semper mihi pārētōte! Dīligenter audītōte! Pulchrē recitātōte et rēctē scrībitōte!"
p. 225–226, quoting Matthew 14:26–27:
Discipulī autem videntēs eum super mare ambulantem turbātī sunt dīcentēs: "Phantasma est!" et prae timōre clāmāvērunt. Statimque Iēsūs locūtus est eīs dīcēns: "Cōnstantēs estōte! Ego sum. Nōlīte timēre!"
I think this is from a version of Matthaeus in use in 150 A.D.; see here for a similar one. Jerome's version has habete fiduciam rather than constantes estote. However, a quick Google search suggests that constantes estote and similar are common expressions. Constantes estote et videbitis auxilium Domini super vos, Estote fortes in bello, etc.
Quārē dubitāstī? :)