No, I think it's related to devtools. You can check that devtools:::version_info is lacking 3.5. You could use this as a workaround:
library(devtools)
assignInNamespace("version_info", c(devtools:::version_info, list("3.5" = list(version_min = "3.3.0", version_max = "99.99.99", path = "bin"))), "devtools")
Now your install commands for any package that needs compilation will work again, assuming you have version 3.5 of it installed from https://cran.r-project.org/bin/windows/Rtools/.
See here for reference: https://github.com/r-lib/devtools/issues/1772
