You have to dig through earlier editions to learn more
The earliest reference I can find comes from the AD&D sourcebook Dwarves Deep (page 4):
Moots are business meetings between dwarven clans or professions, or between dwarves and nondwarven traders. Current known moots in the Realms include periodic Tradesmoots near Baldur's Gate, the annual High Moot northeast of Waterdeep, and the Deep Moot in the Great Rift, held every ten years and open to every dwarf.
Dragonmoots are a proud but vanishing tradition, in which bands of adventuring dwarves are called together to fight specific dragons, and plunder their hoards. They were once something of a ritual of passage for young dwarves aspiring to be warriors.
The most recent (and thorough) reference comes from 3.5 edition's Dragons of Faerun (page 8):
The proud but now-vanishing tradition of Dragonmoots is much sung of when dwarves empty tankards. Of old, Dragonmoots were “called” (by messenger-dwarves sent from one dwarf clan to another) whenever the leader of a dwarf clan decided that a particular dragon was too bothersome to be left alone any longer and too powerful for a clan to risk tackling alone.
Most Dragonmoots involved two or three dwarf clans, the elders of all meeting to decide just how a specific dragon would be fought, and how its hoard would be divided after victory (and such forays almost always ended in victory). Adventurers from the ranks of the clan would be asked to lead the attack, bolstered by the axes of old dwarves desiring to die in battle and the frightened strength of younglings wanting to prove themselves worthy, in the eyes of their clan, of becoming warriors.
Clan elders encouraged Dragonmoots as occasions when dwarf clans would meet, feast together, discuss trade, cast eyes on eligible dwarves of other clans, and “take measure” of each other (marking the likenesses and characters of important dwarves for future diplomatic use). It took as many as seven clans to defeat some wyrms, and more than one dragon ravaged those sent against it.
Dragonmoots began to fade when dwarves grew too few to dare openly challenge dragons (except as members of “crazed adventuring bands”), and certain clan elders grew too suspicious of other dwarves, and of the risks of gathering anywhere, to enthusiastically support future moots. However, several clans revived the old practice during the recent Rage of Dragons, since the Thunder Blessing has increased the ranks of their warriors.
Since the latest Dracorage was in 1373, according to The Grand History of the Forgotten Realms, that would mean the last Dragonmoots could/would have taken place about 100 years before the events of the 5e Dragon Cult modules.
