Considering that it will most likely be quite some time before Apple introduces silicon powerful enough to replace the Mac Pro line, you'll most likely see developers keeping support for Intel CPU's up until at least 5 years after the last Intel Mac Pro is shipped.
My guess is that Apple will keep selling Intel Mac Pro's for another 3 years, so something like 8 years of Intel support would be expected from devs. Not sure how long Apple want to keep supporting Intel in macOS however, but I doubt they'll kill support for the last Mac Pro's that quickly.
Also remember, that developers will still have to develop x86_64 code in order to support Windows. Given this, supporting Intel alongside ARM isn't too much of an issue.