The version should have been 6.0.0~beta.1 instead of 6.0.0-beta.1. The final version will be 6.0.0-1 which is alphabetically behind all the current prerelease versions so apt install powershell installs rc2 instead of final.

The convention for prerelease versioning is taken from http://gazebosim.org/tutorials?tut=install_unstable&cat=install

same for RPMs https://en.opensuse.org/openSUSE:Package_naming_guidelines