Technically, this rule can still exist in modern English so there's nothing really odd about the sentence. However, the use of subjunctive mood in this way makes the sentence sound quite formal and a little archaic. Don't rely on simpler English grammar to understand Jacobean English.

It's an archaic form of the subjunctive used for hypothetical situations.

Old English tended to be less strict in grammar and spelling than modern English. You will find a lot of archaic phrases in texts like the Bible, or Shakespeare. This sentence would be considered strange these days.