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Very doubtful that you would be able to run Safari 14 in Mojave. If you look at Safari Technology Preview 109 it specifically states you need at least Catalina to run it. And it is the basis for Safari 14 used in the beta of Big Sur.

Safari Technology Preview Release 109 is now available for download for macOS Catalina. With this release, Safari Technology Preview is now available for betas of macOS Big Sur. If you already have Safari Technology Preview installed, you can update in the Software Update pane of System Preferences on macOS. Safari Technology Preview is currently only available for Intel-based Macs.

This release includes new Safari and WebKit features that will be present in Safari 14.

 
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