You'll be happy to know that it's trending back to a soft-C. I lay responsibility for this at the feet of the US professional basketball team. I've been using hard-C since I visited the Keltic Lodge in Nova Scotia in the 1990s.
I would not go that far? In Scotland, say, Celtic Football Club hold considerable mindshare, and that is pronounced with a soft-c - presumably because the club was founded before the shift to hard-c of other uses.
But there's no noticable use of a soft-c when people refer to the people/ethnicity. Events like "Celtic Connections" are still with a hard-c. I have not noticed the use of soft-c at all, outside of sporting clubs of various sorts. I don't think that's a trend, it is just yet more of the inconsistency and irregularity that human languages are plagued with.
