The distinction in meaning still technically exists but colloquial usage doesn't really care and just picks one. You can still find plenty of guides online detailing which is more appropriate but more modern articles are beginning to include paragraphs such as the following from Grammar Girl:
The good news is that in ambiguous cases it doesn't matter which word you choose. Although careful writers will try to stick with the distinction between "further" and "farther," the Oxford English Dictionary, Fowler's Modern English Usage, and a number of other sources say that, in most cases, it's fine to use "further" and "farther" interchangeably, especially when the distinction isn't clear. People have been using them interchangeably for hundreds of years.
So, to directly answer your question:
Is it true that 'further' and 'farther' are becoming interchangeable?
They have been used interchangeably for ages in spite of the pedantic distinction between the two meanings. So, they aren't becoming interchangeable; they already are interchangeable.