For people looking for how to disable hard mode before the option is added:
First, make sure you have UndertaleModTool installed.
Now, open the game's data.win by double-clicking on it (duh)
After it finishes reading the file, open the Code section and search for "hardmode". You should see something named "gml_Object_obj_hardmode_Create_0". If you do, open it by double-clicking on it.
Now, search for the line "global.hardmode" (it's usually the first line) and set the value to 0.
Now click out of the debug box to save your changes and then close the application (make sure to save the data.win in the same folder as the original one. If windows says that theres already a file named "(directory here)" then click Yes)
And hooray, no more hardmode
(If you wanna turn it back on, just do the same thing but instead of changing the value to 0 change it back to 1)
hope it helped