Humancell:
Where can I learn which boards are "current" and "strategic" vs. those that might have a more limited life time? Is there something that describes which boards might be heading towards the end of their life cycle, vs. others that are more strategic and have a longer life time?
Yes, it's like a zoo and some of the animals are quite older. There is a long history here. This gets more complicated since the production seems to be in batches and between batches there might have been engineering changes due to changing part availability.
Because Arduino is fully open source companies such as SparkFun, Adafruit, and OSEPP can manufacture quality clone products using in some cases yet different support chips. These are supported through the Board Manager and help provide even more choice or confusion.
Humancell:
I came across this when looking at the M0, M0 Pro, and Zero, and trying to understand the differences and life time of them. I'm also curious what others are the most popular selling at this time.
The Zero and M0 Pro include the Atmel EDBG chip which gives single-step debugging with the Atmel Studio IDE. There are other minor differences between these two boards, again based development history. The M0 is a less expensive version that lacks the 2nd USB port and the EDBG chip. My favorite for development is the Zero with the WiFi101 shield. For deployment these can usually be replaced by the MKR1000.