Red Hat has decided to stop making the source code of RHEL available to the public. From now on it will only be available to customers — who can't legally share it. This is very bad news for downstream projects which rebuild the RHEL source code to produce compatible distributions, such as AlmaLinux, Rocky Linux, EuroLinux, and Oracle Unbreakable Linux.
The core difference is that CentOS Stream is upstream of RHEL: it's what will become the next point release of RHEL. It's a sort of continuous rolling beta of the next version of RHEL. Alma, Rocky, and so on, and the former CentOS Linux, were downstream of RHEL: they were rebuilds from the same source code, guaranteeing perfect compatibility (and stability).
Red Hat is still "complying" legally with the GPL in that they do provide the source code to their customers, but that code may not be shared further by anyone else.
It does sound like there may be also a sort of workaround for the downstream distros, but it's not ideal. So we're going to have to see how they respond with their plans going forward.
Red Hat was acquired by IBM in 2019, and apart from this move, there have also been some layoffs made at Red Hat (as with many other global companies).
See Red Hat strikes a crushing blow against RHEL downstreams
From now on, only CentOS Stream's source code is available to all