It's not controversial information that Kindred is the manifestation of death, and death can happen anywhere to anyone, not just on Runeterra, this was made clear during their Q&A, where it was clarified that they are omnipresent.
One could argue that what they meant was ''they are only omnipresent on Runeterra''.
However, during Aurelion Sol's (and later Ivern's) Q&A, it was stated that Aurelion Sol could die one day, and once that happened he would see Kindred.
Aurelion Sol (yes not a god, much less a universal constant, yet still an extremely powerful celestial entity) has a quote mentioning Kindred:
"If Bard and Kindred can pull off wearing a mask, so can I."
It's worth noting that he also mentions Bard in that quote, equating Kindred to the same deal of relevance of an entity responsible for maintaining equilibrium in the universe (Bard).
Now this makes it seem clear that Kindred goes beyond Runeterra however as of late I have seen people comment that Kindred is only a spirit and that it is limited to deaths that happen in Runeterra, I was curious as to where that idea originated from and if it's backed up by any official or semi-official (like the boards) information.