Siemens often keeps archives of older software for existing customers who need to maintain legacy HMI/SCADA systems.

If you're learning, the includes WinCC Advanced. It runs for 21 days, extendable once.

While you cannot legally download the full version for free, there are limited ways to access or trial the software:

While Siemens occasionally offers free Service Pack (SP) updates for existing license holders, the full base software must be purchased through an authorized Siemens sales partner . How to Legally Obtain WinCC 6.0