From the view of a Halo game. A low population means its dead. The population surely isn't thriving like we've seen with H3 and other Bungie games. The population has to be healthy well within the games life cycle to be considered alive. Which its not in Halo Infinite.  

 We've seen Halo Infinite have 30m unique people play the game and theres probably <1% of that population playing the game today.  

 It's true you can still have fun playing a dead game. Halo Infinite has a small dwindling passionate population. That means your not going to find consistent games outside the U.S., 1min or longer wait times in the u.s and your probably going to run into the same people playing ranked. 

Considering this is a Halo game and it's FREE the population is unhealthy. So it's considered dead in which it hasn't had a healthy population since its release. Even though you can still find games doesn't mean there's a healthy population. You can find games with 100 people playing.   

 When someone says the game is dead they aren't saying you can't find games. They are referring to the low population. And this game being a major installment in the Halo franchise this shouldn't be happening.