This update is not a security update per se. However, it contains a check in case of a missing or incorrect application secret and can thus protect a potentially vulnerable installation. The attack would be conceivable for any Symfony application independent of Contao.
About Contao 5.1
The first stable version of Contao 5.1 has been released on 15 February 2023 and will be the successor of Contao 5.0. 5.1 will be updated until 14 August 2023, after which it will be replaced by Contao 5.2.