The term "subspace weapon" refers to any device whose intention is to disrupt subspace via destruction or damage to the fabric of space-time. This definition excludes gravity wells, as those do not cause damage to space-time.
Early on in Humanity's early experiments with subspace, it was noticed that when an experiment failed spectacularly, the area around the explosion would become unstable and difficult to travel in subspace. Even in normal space gravitic waves could be detected, though most of them were harmless.
After some more refinement, a specific device that could detonate in subspace and cause disruption as build and detonated. It caused a disruption that lasted for several days. Larger devices were built and tested until the test site became entirely unpassable for years.
As an explosive, a subspace weapon is devistating to whatever is in it's immediate radius, crushing it like an instant back hole. However, it's the longer-term effects that are truly terrifying. Destroying faster than light travel would simple kill some worlds outright and destroy any possibility for a decentralized government. Most larger population worlds don't have the infrastructure to operate on their own, making them very vulnerable to weapons like this.
Knowing the find of society-ending damage large-scale disruption of subspace could cause, all major factions of Humanity refrained from using such weapons in warfare, though it was known or suspected that both sides had stockpiled such devices. A few isolated deployments were all the use they'd ever seen, and those detonations were small enough the disruption was measured in hours.
(See Battle of Charlemagne.)
During a particularly brutal battle in the Carolingian system, the TRS Bulawayo exploded, setting off a subspace detonation. It is unknown if this was a deliberate execution of a subspace weapon, or simply a malfunction, but the entire Carolingian system has been cut off from subspace ever since.
After witnessing the devistation in the Carolingian system, both the League and the Terrans signed an accord that banned the usage, development, or construction of subspace weaponry. All current subspace weaponry was destroyed under the supervision from the NorAellians.
No other races than Humanity have admitted to building subspace weapons, though there's reason to believe most advanced races have their own version of the Charlemagne Disaster. A cautionary tale that proved to them the risk was simply too high.