If anyone doesn't want birdsite drama on their timeline here, you can add "RT @" and "@\twitter.com" (without the backslash) to your filter keywords. These essentially show up when people crosspost birdsite to the fediverse. Make sure to unselect 'Whole word' matching for this to work and set 'Filter action' to 'Hide completely' to make it airtight. Instances running newer Mastodon releases also allow you to group keywords as in the screenshot below.