Starting with BioC 3.6 we are providing an Archive directory for each release. The Archive contains all package versions that occur during the 6 month cycle. This will be done for the release branch only.
The structure is similar to the CRAN Archives except there will be a separate Archive for each version of Bioconductor. You can see the 3.6 Archive here:
If a package has multiple versions, there will be an 'Old Source Packages' link at the bottom of the landing page, e.g.,
To install a package from the Archive use biocLite():