I noticed something in an older YouTube video. Is it just my impression or was the default line spacing in DT 2 greater than in DT 3?
Older macOS versions used a different system font (or maybe a custom font was used for the video) and the line spacing depends on the font & size. But of course the user interface of v3 was completely overhauled too. Even the line spacing of e.g. List view in v3.9.8 is not identical to v3.0 anymore.
A custom font? Was that possible in DT 2? Not in DT 3, as far as I know.
Anyway, you don’t want to implement adjustable line spacing, which is your right. After all, it’s your app. 