Term::whereTaxonomy('foo')->get();
Term::whereTaxonomies(['foo', 'bar'])->get();

Term::whereTaxonomy('categories')->where('slug', 'projects')->first();

 // second where is collection method, not database query
Taxonomy::where('taxonomy', 'categories')->get()
    ->pluck('term')->where('slug', 'projects')->first();

// code copied from this PR
Term::whereHas('taxonomy', function ($query) {
    $query->whereIn('taxonomy', 'category');
})->where('slug', 'projects')->first();