I'm not aware of a sale on the Skill Unlock Pack from memory.
The last sale that happened on the Games was Nov 2019 (Fall/Black Friday).
The add-ons like the Character Slots, Xunlai Panes, Mercs etc. last went on sale April 2020 (GW 15th Anniversary).
Since it has been over a year since any sale an assumption could be made not to expect any more sales. Specially since the Anniversary event just gone would have been a good time to do it.
Buy some Flames of Balthazar to get factions, then use them to unlock skills at the Priest of Balthazar in guild hall/Gtob. It will unlock those skills for pvp characters and all heroes for pve characters. Those packs were never on sale.
