Its really a recommended feature on the free plan, so we can purge for example https://blog.example.com instead of the entire zone. And still Cloudflare insist us to not purge everything in a zone…
Reminder: We highly recommend performing single-file purges.
You generally seem to request a lot of features which are on paid plans for the free one. I am not sure this is the overall intention of the different plans and I guess if Cloudflare would have wanted to offer them on the free plan, they’d be already on the free plan