Outdated: usually because a flavoring has changed. A couple years ago when FA Meringue changed, I had to redo lots and lots of recipes. Some still worked; some were changes; some just didn't work any more. Sometimes a new, better flavoring becomes available... Flavor West is constantly changing flavor names & the actual flavor, so most of my old recipes that used Flavor West can't be made any more & are thus outdated.
Retired: maybe because it just wasn't very good, or I like it but others didn't, or it was good in cartomizers at (what was that, 3.7 watts? lol) low power, but didn't work as better hardware became popular. Some of my earliest coffee recipes used TFA flavors, for example. A couple times it was out of respect for someone else's recipe that was similar but predated mine, or someone else just plain did it better.
Replaced: that's often a recipe where non-FA flavors were replaced with FA to make something better or different & avoid diketones. I'm always on the lookout for ways to get better flavor without those ingredients.
Briefly on the flavor-component warning radar for a while was the combination of ("naturally occurring," since no manufacturer would admit to adding) diacetyl plus acetyl propionyl. A study showed the combo was potentially worse for lung tissue that the same quantity of diacetyl. I zapped my recipes with the combo of larger quantities of Nature's Flavors ("naturally occurring" diacetyl possible there) AND TFA flavors documented to contain acetyl propionyl at the time. I keep an eye out for studies; haven't seen anything more on that combo, but I've never seen it contradicted, either.