Suppose I have a multiclassed bard/warlock who chooses unseen servant as one of their warlock spells. Given that unseen servant is also on the bard list, can they then use the bard's Ritual Casting feature with this spell?
The Ritual Casting feature says
You can cast any bard spell you know as a ritual if that spell has the ritual tag.
The cleric and druid Ritual Casting features use equivalent wording (wizard is different). The question is, what defines a "bard spell". There seems to be no explicit definition.
The best clue I can find on this terminology is in the wizard's spellbook description
When you find a wizard spell of 1st level or higher, you can add it to your spellbook...
This applies to spells found on spell scrolls as well as spellbooks. Now, a spell scroll doesn't have a class attached: whether a cleric, wizard or sorcerer scribes a detect magic scroll, the result is the same. So in this quote, "wizard spell" must mean "spell on the wizard list". (See also this question: If a cleric wrote a scroll of a spell that a wizard can learn, could a wizard learn the spell?)
If "bard/cleric/druid spell" is used in the same way in the Ritual Casting rules, then a multiclassed bard-warlock should be able to cast a warlock unseen servant as a ritual. But it would be nice to have some more support for this conclusion (or counter-evidence).