It seems that as of April 28, 2018 the R 3.5 is not available for Linux in any of the mirrors. The latest r-base-core is for 3.4.4, see the image below from one of the mirrors. Meanwhile I was able to install R 3.5 for Windows. Do you know the reason why the latest R is not available for Linux?
Do you know the reason why the latest R is not available for Linux?
Because we are volunteers.
Moreover, R 3.5 breaks the binaries of R 3.4.* and older so we have to rebuild everything first. That is work in progress (for what is mirrored at CRAN).
For Debian proper, I of course uploaded the new packages the morning of the release (as you can see in the ChangeLog) but we are now in a so-called transition where all packages need to be rebuild (which you can monitor via this tracker).
I am thinking of making interim packages available somewhere else. But I would have to find the time to build them first too.
Edit: I expanded a little in this r-sig-debian mailing list post.