Not necessarily a permanent solution for all platforms and microsyntaxes depending on the number of syndicated copies, but potentially a clever stopgap for those who may need it.
One can use a zero-width space (using something like ​in their HTML) between the @ and a twitter user name on the original post and the syndicated copy will not have the traditional @mention link or notification functionality.
Here’s an example

Original post: https://boffosocko.com/2020/09/17/55777062/
Syndicated copy: https://twitter.com/ChrisAldrich/status/1306748549233815553

This reply can also serve as a test for the functionality within Github where I’ll “tag” both @kylewm and @​snarfed, but if it works, Ryan shouldn’t be auto-linked or notified.