I have seen examples where the subjunctive in the second and third person is used in both the affirmative and negative sense to make a more ‘polite’ order or request but isn’t this construction more commonly found in medieval Latin?
This is what Woodcock, A New Latin Syntax, sec. 126 Note (i), has to say:
The use of the second person of the present subjunctive instead of the imperative, e.g., > taceas > for > tace> , ‘be silent’, is common in early and colloquial Latin, but rare in classical prose. On the other hand, it is regularly used, even in classical Latin, in addressing an indefinite second person, i.e., in giving general instructions or precepts.
H. P. V. Nunn, An Introduction to Ecclesiastical Latin, p. 43-44:
- The Imperative Mood is used to express commands and entreaties in the second person singular or plural, and has forms which may be used to express a command given in the third person.
(…)
In Eccl. L. the second person of the Present Subj. is used to express a command or entreaty.
Nec doleas, quod talem amiseris, sed gaudeas, quod talem habueris> . Jerome, > Ep. > 60