Whether racing or cruising, keelboats are expected to be equipped with appropriate lights; these are required both by the Collision Regulations (ColRegs), and our SIs in the case of racing.
There is nothing in our Interclub Sailing Instructions (SIs) that precludes keelboat races from being run after sunset, and sailors should be prepared for that as the days get shorter in September. If sailors are uncomfortable returning to harbour after dark, then they should check the time for sunset, and plan accordingly.
Races are started 15 minutes earlier in September, due to the shorter days, and in most cases, races will be finished by, or shortly after, sunset, even in the latter half of September, when the sun sets between 7:10 pm and 6:45 pm. However, races can be postponed, or the wind can die, and it is not uncommon for races to continue after sunset, and for boats to be returning to the harbour in the dark.
Be sure you are prepared.