People keep saying that Sans was originally from Deltarune is "basically canon" is so weird to me considering one of the first things Toby established about Deltarune was that these characters and this world isn't related to Undertale.

"Essentially, this is not the world of UNDERTALE.
If you completed UNDERTALE, the ending and world are as you left it.
If everyone was happy there, the people in the UNDERTALE world will still be happy.
So, please don't worry about those characters, and that world. It will remain untouched.

To rephrase that, DELTARUNE's world is a different one.
With different characters, that have lived different lives."

Toby loves messing with the player but he never just outright lies to pull one over on them. It's much more likely that Undersans talking about missing a "previous life" is just him lamenting over the things he loses every time there's a reset.