How likely is it that hiring apps and workplaces discriminate against leftists

Unlikely - the further up the scale of education and responsibility you go, the more scarce rightoids get.

Workplaces definitely do screen people based on social media which can include how politically they are publically

Would not surprise me. Amazon already got caught using AI for union busting.

It's honestly more likely that right wing Republicans are being screened out by such apps for certain professions.