The fandom's reductionism towards Undertale in relation to Deltarune.
I often see people being reductive and undermining Undertale's importance, talking about how "Undertale was just a prototype for Deltarune" and how Deltarune was Toby's "real vision" all along.
Undertale IS part of Toby's vision for Deltarune, and is an equaly important game as Deltarune.
Undertale wasn't just a thing Toby did so he could later make Deltarune, it's a foundation that allowed Toby to make the current version of Deltarune that people love.
Undertale was Toby's first full original game. The first time he got to develop a full original world, with a complete story, characters, and gameplay. All of that became a main foundation from where Toby developed Deltarune from.
All of the returning characters that fill up Hometown and their relations with the new characters introduced in Deltarune, the main gameplay and battle structure, even some ideas and characters that came to be because of their relation to stuff from Undertale. All of that got to exist the way it does now due to Undertale existing first as a full game and story experience.
I feel like people overplay just how planned out Deltarune was before Undertale. Because before Toby began working on Undertale the ideas and characters planned for Deltarune were all pretty limited and scarce. Then as Toby was working on Undertale those ideas and characters began solidifying more as Undertale was already influencing Deltarune. But Toby only began seriously working on Deltarune's story and properly stablishing the characters beyond the basics after Undertale was already complete.
In fact, every other Deltarune Chapter beyond the first one only began being thought of at around 2016, after Undertale had already released. Stuff like Spamton NEO, Tenna, and Flowery were thought of in relation to Undertale.
Spamton NEO began life as a follow up to Mettaton NEO, Tenna was created to be like Mettaton, Flowery existed in reference to Flowey. In fact, I don't know if the 7 Colored Flowers would even exist if Undertale hadn't stablished the fallen humans.
Needless to say, Undertale was pretty necessary for us to get the Deltarune that we know of today. But that's not to undermine Deltarune as its own thing, far from it.
Everything in Deltarune is able to exist on its own, despite everything Undertale did to influence it. Deltarune has droves of original ideas, worlds, character arcs, and music specifically made for it. Many of it even surparssing Undertale in execution.
But I just can't stress enough how important Undertale was to allow for all of that to happen. Because otherwise, Deltarune either wouldn't exist, or would have existed in a very different form (a form I'd argue wouldn't have been as good as the one we have now).
Undertale is kinda like Super Mario Bros. (NES). You could argue that it is a "prototype" for all of the later Mario games, but the game absolutely needed to exist to serve as the necessary foundation for everything that came after, both new things and the things that were stablished in the first Super Mario game.
(You can even imagine pre-Undertale Deltarune as being the pre-Super Mario games, like Donkey Kong and Mario Bros. (Arcade).)