In the game Battlefield 1 you can fly a Sopwith Camel vertically and perform different kinds of aerobatic maneuvers like loops. According to its specifications, its thrust-to-weight ratio should be about 0.28 and hence it should not be able to fly vertically. Can it?
With thrust/weight ratio above one, you can use the thrust of the engine to accelerate upward (at least at some point in the flight envelope). As long as the engines can deliver full thrust, you can maintain that position.
Below one, vertical flight is still possible, but you will be losing speed in the maneuver.
Gliders are capable of loops without any thrust at all. The only energy in the maneuver is the speed of the glider on entry.
Yes. Plenty of airplanes with thrust to weight ratios less than 1 can perform vertical maneuvers though time in high pitch or vertical lines is limited as airspeed drops rapidly. Often such airplanes have to be placed in a shallow dive to accelerate them to an appropriate entry airspeed for the maneuver. This is sometimes true for aircraft which have a thrust weight ratio greater than one as well, as, while a thrust to weight ratio > 1 provides an expanded maneuvering capability in the vertical, the aircraft is still slowing down as the combined force of the weight of the aircraft plus drag on the airframe are generally greater than engine thrust.