Translation:These are the ideas that we thought of yesterday.
I was confused with the same thing as you, but I've come up with an explanation. The relative pronoun "auxquelles" actually represents an indirect object "aux idées", which is different from the relative pronoun for a direct object (que). When it's a pronoun for direct object, the past participle agrees with the pronoun preceding it. But when the preceding pronoun is for indirect object, the past participle does not.
For example, when the object is direct, we will say "j'ai abondonné les idées / je les ai abondonnéES / ce sont les idées que j'ai abondonnéES". In contrast, when the object is indirect, we'll say "j'ai pensé aux idées / j'y ai pensé / ce sont les idées auxquelles j'ai pensé".
Hope my response is helpful for you.
i have to disagree. "to think up" means "to invent", "to come up with [something]". it was a translation hint because in certain contexts it would work. but here, the preposition "auxquelles" shows that the ideas are something you think about, not something you thought up. what you're suggesting would probably be "les idées que nous avons pensées" (also, notice here the accordance of the past participle of "penser" with the direct object "idées")
I think the most natural expression of this sentence would be; 'These are the ideas that we were thinking about yesterday', but since the sentence uses the perfect tense AND auxquelles the closest translation would be; 'these are the ideas about which we have thought yesterday'. One of Duolingo's allowed translations - - 'These are the ideas about which we thought of yesterday' is quite wrong in English, you don't use both 'about' and 'of' - unnecessary repetiton.