People who have played franchise mode in Madden will know that once you’ve played even a season or two, some of your older players have lost some strength and speed because the years of punishment are finally catching up to them. Are there any romhacks built around this concept? Effectively, gaining XP is akin to aging, so units would slowly get worse just from wear and tear.
There’s a few patches for vanilla FE games that effectively replace stat gains with stat losses.
That being said, I don’t think there are any fully custom hacks with that design philosophy in mind. Unless someone wants to come along and make that a reality some day.
Tequila made a Make a Fun Chapter entry using this as a gimmick a few years ago. Level up, and down your stats go. There are no full-length hacks like this, though.