When monitoring the buildbots for Master and 18.06 I tried to find a link for the 18.06 snapshots on openwrt.org but couldn't, however a google search came up with this
To my knowledge, yes.
Release branch buildbot builds are meant for testing, not for regular usage by casual users.
There are the proper rc and release builds that are meant for wider population usage.
Because at some point in the past we deemed it too confusing for users to publish something like "release snapshots" which are a moving target wrt. compatible kmods etc.
Actually yes, it is some kind of a transient location. The builders dump their release branch head test builds there. Strictly speaking they should / could go into snapshots but that would require logic changes to the buildbot to choose the rsync destination depending whether a tag is getting built or just a random stable branch commit.
While this is not exactly rocket science to implement I was too lazy so far to pursue that.