'brick' in general is a sliding scale based on how many device debug/recovery tools you have access to and how much device disassembly you're willing to tolerate.

No, iOS devices can also soft-brick, it's just a general term. Most often people encounter the term in relation to hacking/rooting hardware, like a Nintendo DS/Switch for instance. Hitting up Wikipedia or Trends can help you dig through its usage considering how Google's search algo heavily biases current events wrt relevance.

Not really, it's used for example with OpenWRT too:

site:openwrt.org "soft-brick" - 213 results