The modern trend is to lower these minimum length requirements, especially when non-vulnerable. Conventional preemptive openings used to introduce a weak single-suited hand include the multi 2 diamonds and the gambling 3NT conventions.
In bridge play, there's a shift towards lowering minimum hand length requirements for non-vulnerable hands.  Historically, common preemptive openings for vulnerable hands involved using the multi 2 diamonds and the gambling 3NT convention.