The Dalish forgot, but why?
I have a feeling that the Dalish didn’t *forget* just because of social and historical processes. Solas maybe had his hand in that too. One last thing he could do for the survivors who would now have to live under the Veil, suddenly alone and disconnected. Spare them the trauma of remembering slavery under the evanuris and the bloody fall of their empire. Spare them from the pain of remembering everything everything everything they lost and cannot regain. In hopes that it would set them free.
It really does sound like something he’d do:
“I need to do a thing to fix a thing”
"I need to carry this burden alone”
“I’ll do a thing for them without consulting them, it’s the only way”
“OK that’s done, nap-time”
*wakes up*
“Ugh that didn’t turn out well either”
“Hey more crushing feelings of guilt”
“I better push them away and imagine they’re not people that will help”
(I can’t find it, but early in Inquisition, but during that first ‘elven culture’ discussion, there can be that bit where Solas asks the Herald how it would help an elf starving in the alienage to know his ancestors once ‘strode this land like gods’ - or something like that - and then the Herald can call him out for deciding their reactions for them.)