I’ve seen many images of older planes, notably fighters and bombers from the WW2 era have string/wires connected from the top of the plane, usually suspended by a stick or something, connected to the tip of the tail.
What was the reason for that, and around when did we stop suspending the tails with wires?
Rather than structural bracing wires, it sounds very much like you're describing radio aerials. The lower the frequency of radio transmissions, the longer an aerial needs to be to receive them; the logical location for a long wire aerial on an aircraft is from a mast behind (or even in front of) the cockpit stretching to the top of the vertical stabiliser. With higher frequency transmissions, shorter aerials can be used; early Supermarine Spitfires and Hawker Hurricanes used the TR9 High Frequency (HF) radio and had a wire strung from mast to tail. Once the Very High Frequency (VHF) TR.1133 set was introduced the aerial mast behind the cockpit alone was sufficient, so there was no need for the wire aerial. Spitfires and Hurricanes could also have wires emerging from the fuselage to the tips of the tailplane for early Identification Friend or Foe (IFF) sets, though again these were superseded by improved designs working at higher frequencies that only needed short underwing aerials. See The Spitfire Site for an illustrated article outlining the various masts and aerials, or the Tailhook Topics blog for an extensive piece on F4U Corsair aerials or a more general overview or carrier aircraft aerials.