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Devs share the most popular multiclasses
(live.staticflickr.com)
Everything about original Baldur's Gate series, Enhanced Edition and the Infinity Engine games.
Rogue Multis are going to be popular due to how good Thief 3 is. But for this particular combo it's most likely a hold over from table-top where Gloomstalker Assassin is quite powerful. It still is in BG3, but it isn't on the same par as a Thief Swords Bard or any of the other broken combos.