Soo i wanna say it's semi clear at this point that the general story points to some kind of timeline reset/universe shift/however you wanna call it. Where the universe changes to Undertale and Sans is the only surviver/aware one trying to come back to the Deltarune timeline.
Just way way too many things line up and Toby is not dumb enough to do those things by mistake. and after all he was working on Deltarune way before everything else.
>The notes in the lab behind Sans's house. clearly working on some universe level device.
>The whole "place where it rained" connection between both games.
>Sans mentioning several times of missing people and times
>In the ending section where the characters insecurities are revealed (idk how to call it. Where all the characters have the face blur). Sans is all about being worried of never coming back and it all being futile and all that type of stuff.
>General vibe of Sans clearly being melancholic in UT while just nicely chilling in Deltarune
>"Don't forget" and old pic where Sans is happy.
I think there is a few more but I forgot, tell me if you got more.
Personally I hope that's not the case, I just don't really want the story to go that way imo.
Also on a note it might be the case that UT is actually BEFORE Deltarune and he did manage to shift back and we are seeing the aftermath for him(also few hints that this might be the case.)
But you tell me anons?