This thread makes me feel old already, since I lived 3rd gen peak, talk about all those distinguish acts of that gen like stars of the past is troubling for me. You can have any definition about what a legacy act is but for me it doesn't depend if a group is officially disbanded or not, or still releasing music sometimes, a group is a legacy act when they are not relevant enough anymore to be in the spotlight and are seen as a thing of the past, whatever they do is mostly limited to their circle of fans, nobody is checking out their music, new kpop fans are not familiar with them and not interested in becoming their fans.
For example since SM never disbands their groups super junior could as well never officially disband, hell they have probably made more comebacks in the last 10 years than a lot of 3rd gen groups but by all means they are totally irrelevant group who barely anybody checks out, and new kpop fans may recognize them a little since they are an important part of kpop but is just that, "hey is that group who was very popular 15 years ago". And the sad reality is, majority of 3rd gen kpop groups are already in that state, getting less streams than acts that have barely been a thing since a decade like girls generation or super junior. I will use youtube views to determine this, is the platform that was widely available since the 2nd gen and still one of the principal ways to consume kpop content.
Youtube Streams Range
Girls Generation: ~400K
Super Junior: ~200K
Exo: ~500K
Got7: ~100K
Mamamoo: ~300K
Monsta X: ~90K
Winner: ~100K
Momoland: ~100K
iKon: ~200K
Gfriend: ~100K
Source: Kworb
All of those acts were some of the most popular kpop groups around just 5 years ago, is terrific to see how damn fast things change on kpop and groups become things of the past, with some of the best bangers and biggest hits on kpop history getting dust barely being watched anymore like decalcomanie, overdose, ice cream cake or rough. And if this is the state of all those very popular groups of 3rd gen imagine how the lesser popular ones state.
But that's life my friends, sooner or later all our faves will be legacy acts, just another page on kpop history and only the memories will remain, but I can't lie, is shocking that those groups are already there with some releasing music not long ago, 3rd gen of kpop is over and you can do nothing about your faves being legacy group, the sad reality. And in no time some groups like red velvet, nct 127 and nct dream will be there too, they just have the SM never disbanding their groups thing in their favor, but they are on the edge already.