Where has Frontier stated this?
There was nothing like this on the Horizons Beta store page https://archive.is/y8oDc and I see a moderator here has said (for what it is worth) "You bought access to the Beta phase of Horizons, which is over. "
They did not say it but last year, all major updates ran their own individual beta phase before they made it into the release. Wings, CQC, Powerplay and 1.5 all ran separate betas. Why would it be different this time?