Bamboo only keeps last 10 builds, so only builds 623-632 are available now 
You can always check for the latest build yourself by going to http://bamboo.igniterealtime.org/browse/SPARK-INSTALL4J then pressing on the latest build number (#632) -> Artifacts -> install4j -> then the first link with the name spark_2_6_3_12555.exe
So the current latest version link is http://bamboo.igniterealtime.org/artifact/SPARK-INSTALL4J/JOB1/build-632/Install 4j/spark_2_6_3_12555.exe
2.7.1 isn't even planned yet. 2.7.0 is still not released. So this link is the only latest version. You can say "Beta". Though it is more a living thing, constantly evolving, changing. Every build is the new patch applied, new fix, sometimes new bugs. #632 seems pretty stable. I'm using it now on my network with ~200 users.