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The most significant danger of searching for "Microsoft Office 2019 product key GitHub" is not legal; it is cybersecurity.
Microsoft takes this seriously. While they rarely pursue individual users legally for a single installation, they have every right to deactivate the software remotely. If Microsoft detects that your installation is using a blocked key or an unauthorized activation method, they can render the software unusable or constantly nag you with "This product is not genuine" notifications.