Have single length iron sets been done before?

Yes.

A Canadian company by the name of Iso-Vibe designed a set a number of decades ago. Tommy Armour came out with the EQL’s in the 1980s. My Ostrich Golf created the PureFit iMatch SLs but are also no longer in production.

More recently, 1Iron Golf, Value Golf, GRIA Golf, Edel Golf, Cobra Golf, and several others have manufactured single length irons.