No. Modifying the game AT ALL, unless SPECIFICALLY allowed by the community is banned. This is site-wide.

Deltarune does not allow this, so modifying the data.win file in ANY way is banned and will get your run rejected.

According to SRC rules: "The game itself must not be altered or controlled with outside tools. Exceptions can be made on a community basis (e.g., character creation scripts, logo/cutscene removal dlls, etc.)"

No exceptions have been made for Deltarune, thus modifying the data.win regardless of purpose is banned.

"Let's say I modify a string that's related to a textbox that only appears in an area that I don't visit at all in my run. That's technically a violation of the rules, but since the modified string never shows up in the run itself, the mods will never be able to tell."

The mods will never see the change so it would be impossible to verify so someone could do it and we would never know.

"And this applies to the sprites too. Unless the height/width of the sprite is modified (therefore altering the chance of wall-bumping), I personally see nothing wrong with sprite modification."

The main reason this is banned is to keep the game as vanilla as possible. Another thing can also be changing certain sprites to make certain tricks easier and such like creating red pixels i.e in rooms to help with lining up tricks for example. You could argue this with music mods as well however this is significantly more difficult to take advantage of and the general positives to music mods outweigh the negatives.

this is a type of idealized reasoning that assumes all people involved in modding and verifying are perfect aware of everything that might happen thing, which is not true (ever). the oggs were allowed under the assumption that we knew enough about it, and suddenly, we find out that in CH2 it has effects to gameplay. So allowing this to just about anything is a quick path to headache, for no good reason

to add to music mods, the music mods that are known to affect gameplay are banned outright anyways. cybers world has the tiles that hit the ground based on music, and pandora palace has the laughing that stops the paintings from shooting fire at a different time, which messes up a cycle and makes it different.

there are people that disagree with only banning these two, as it makes it easier to have them all banned, and its fine to have that opinion. what no one agrees with is not banning those in any way, since they change the game fundamentally.

finally, music mods are allowed BECAUSE they arent edits of the data.win. theyre just files in the game the game reads, audio files, very easy to edit, aside from cyber battle which has its own unique file thats banned to be editted outright since its a data file and not really an "audio file."

It's generally better to take serious rules proposals to the discord rather than posting it on the forums, since the discord is where all of that is discussed (and voted on, if needed). But there are very good reasons to ban data.win modifications completely and very little benefit to reversing that.