in 2021, here is a summary of the consensus from automation professionals and official documentation. Direct Verdict TIA Portal V15.1 is widely considered the most stable "legacy" version
In 2021, Siemens piloted a cloud-based TIA Portal instance. You paid for compute hours and accessed V15.1 via a web browser. This eliminated the need to download 20GB altogether. best 2021 download siemens tia portal v151