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.

Straight from the Qt (link above):

> 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.

So these mirrors don't exist then? https://download.qt.io/static/mirrorlist/

Again: to 5.15.x after 5.16 or 6 is out.

Or, in more words as Qt pusts it:

> 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.

It has nothing to do whether mirrors do exist or not, when there will be nothing to mirror.