I suggest to move single file support requests to plugins and create a FR to define a “user-customizable extention for the notes files”.
It can be either .md or something else, but it can’t be a mix of extentions because in obsidian the filename has to be unique and we would have problems handling where [[note title]] points if there is note title.md and note title.txt and both are valid targets.
IMHO, This is gonna be a low priority FR because the supporting arguments for .txt are weak.
Both window search and apple spotlight index .md files just fine.