Did the owl statues really not exist at all in the Japanese versions of The Legend of Zelda: Majora's Mask?
I've been reading this: https://tcrf.net/The_Legend_of_Zelda:_Majora's_Mask/Program_Revision_Differences
In the Japanese versions, there are three save files, whereas the American version restricts you to two. This, however, comes at a price: saving at Owl Statues is not possible at all in the Japanese versions, so you were required to use the Song of Time every time you wanted to save the game. Apparently, that turned out to be really annoying, so Owl Saves were added to the American version which necessitated removing the third save slot.
And:
These [owl statues] were added to accommodate the new system of saving at Owl Statues that was added for the American release.
Did the original game not have owl statues at all? I thought that those owl statues also indicate the spots where you can "soar" (teleport) to. So in the Japanese version, you just "soar" to random locations with no object indicating the "magical spot"? Or are they there, just that they cannot be used to "save and quit"? If so, the page linked to is wrong.
Why, as it says on the page, would having owl statues "necessitate" the removal of the third save slot? That makes no sense to me.
The Japanese version of Majora's Mask does have Owl Statues, it's just that you can only soar to them and not save there. (In fact the speedrun of Majora's Mask is done on the Japanese version and involves significant use of Owl Statues.)
You're slightly misunderstanding the information about Owl Statues at TCRF. Since you're looking at the "English-Only Text" section in the page about Majora's Mask changes, the sentence should not be understood as:
These [owl statues] were added to accommodate the new system of saving at Owl Statues that was added for the American release.
But instead as:
These [two pieces of text that are shown below this sentence] were added to accommodate the new system of saving at Owl Statues that was added for the American release.
Only those two pieces of text about saving games had to be added.
As for the reason for removing the save slot, the exact details of how much space is involved is something you'd need to get inside the game to figure out, but the short version is that having up to two saves stored for each game file (Song of Time save + Owl Statue save) and three game files mean you could have a total of six save files needing to be stored at once and this takes up too much space for them to be able to add it in after the fact. Two game files means a maximum of four possible saves at once, which isn't too much more space than the Japanese version's three.