A lot of agility training is going to be tied to the training and drills that are done for a specific sport. Whereas training things like strength, power or conditioning is typically done more separately. So you'll find more information on training strength, power, or conditioning as a general athletic quality, but tend to find information on training agility tied to reading about training/coaching for a specific sport.
Which makes sense, as "agility" involves specific movement skills, which will vary depending on the sport, and is incorporated well into sports specific skill work, unlike S&C work.
That said, there is some research supporting some kind of agility work for general health and wellbeing among older individuals, being useful in addressing things like osteoporosis and risk of falls.