Nowadays there is little talk about POA, because it is no longer commented on in the networks? What caused its "disappearance", if it is not obsolete, where it is being applied.
The projero Roo from spring.io strongly uses AspectJ. But I've also never seen people using Roo.
Is it worth studying this paradigm (yet)?
I can not tell if it's actually less spoken, I do not know any specific research on this. However, AOP has not disappeared and there is a lot of talk, just see the number of questions in the aop tag in SOen by searching by aop also in SOen.
If your biggest interest is in Spring projects, you can start with here
In Spring as a whole AOP is heavily used. When you use Spring MVC, Data, Social, Transaction Management, etc. every part of it is using AOP in some way, even if you do not perceive it so clearly. For example, just changing in a class from which the object (s) are managed by the Spring context from @Component to @Service (and vice versa) will have differences
In Java EE AOP you are also quite present, look for example interceptors , just talking about using java technologies, here you can see a list of support on some platforms , whether native or not.
How much is worth studying, it depends, AOP is a paradigm that coexists with others, you can someday need it, you can not, so you can not tell if it still compensates or not, it depends on context.
In many cases quite useful; in others, quite obscure. Just use Spring projects and think about where this is, as logging is largely done with aspects, which can make it difficult to follow execution flows in certain cases.
