(Arq developer here) Haven't gotten many requests for it at all over the years. I presume it's because there are so many free options for Linux.
I for one would pay for Arq on Linux as I now do on Mac. It would be fantastic to be able to use the same "it just works" backup solution on all my computers.
Just wanted to say that for many years, Arq has been my backup solution. It's amazing and I advise it to everyone I know.