That's incorrect. The 5.15 branch is and will be public. Most if not all changes to that branch will go via the dev (Qt 6) branch, which is also public.
> Starting with Qt 5.15, long term support (LTS) will only be available to commercial customers. This means open-source users will receive patch-level releases of 5.15 until the next minor release will become available. This means that we will handle Qt 5.15 in the same way as e.g. 5.13 or 5.14 for open source users.