Let me start by proclaiming with clarity and sincerity: No, Safari 14 (or any other version of Safari) will not block Google Analytics from loading and running on a website. In the midst of Apple's yearly Worldwide Developers Conference, the company showcased some of the privacy improvements to the upcoming version of the Safari web browser (version 14). In fact, the biggest revelation was the new Privacy Report, which is designed to elucidate how much the browser is working towards mitigating the damage caused by cross-site trackers.
With Intelligent Tracking Prevention, the Safari browser is on a crusade against cross-site tracking. One of the most obvious and long-standing ways to battle cross-site tracking has been to block third-party cookies in the web browser, and this is exactly what Safari does by default. However, Google Tag Manager's Preview mode relies on a third-party cookies, so that it can serve you the draft version of the container while serving the regular, live container to your site visitors.