Is it worth my time?
About twenty-five centuries ago Hippocrates of Kos famously stated that life is short, while art is long. Most knowledge workers can very much relate to that. While the body of knowledge in a field is ever growing, the time available to an individual for deliberate learning activities is limited severely.
I use the following criteria to assess if a topic is worth spending my time on it.
Longevity and Transferability
I prefer evergreen knowledge and focus on topics that are not bound to a specific technology. As a general principle, anything with a version number attached to it should be treated with caution. But: Using a specific technology (even if it's non-mainstream or dated) to illustrate a concept is fine.
Applicability and personal usefulness
Many concepts are long-lived and transfer well, but when the chance of applying them is rather slim, then other things should take priority. Not applying something is detrimental to retention and time is too valuable to spend it on learning something that falls into oblivion.
As a caveat: To some degree, it cannot be known which topics will lend themselves to be applied to problems presented to you due to external needs and interests. Chance favours the prepared mind.
Not everything which needs to be learned will be a beautiful timeless idea. Sometimes I need to grasp a poorly documented functionality of some obscure technology to get a task done. It comes down to a judgement call, which is a matter of experience and confidence, how deep to go. It can be beneficial to build a real understanding instead of just copy-pasting from the web (which might get the job done but comes at some loss of control).
Personal interest and commitment
There are also a few topics and technologies, to which I'm committed deeply. I consider it worthwhile to keep up with them, even if the criteria laid out before are to some extent not met. I also regularly reassess my commitment to the topics.