NPC sprites have always been part of our "would be nice to have" checklist, for things that we didn't have resources for (time and money, in this case) but wanted to do, but we also don't like making promises we can't keep, so we haven't been promising to fill them in. That's probably why you're surprised. But right now we're in a sweet spot of development where Mugi doesn't need to fully focus on making art for the new chapters (since they're still being written), and we've got some money thanks to everyone's kind support, so we figured we'll go for it.
Adding a toggle is a possibility (it would just darken the new sprites until they're fully black, which pretty simple to program), though I don't think we'll go back to a world where literally everyone but the love interests is a silhouette. It's like you said: it makes everyone feel like a faceless stranger, which is great for the character you meet during the parties on Oleander's route, but less appropriate for the Mouse Hole characters on Keir's route.