Not about plugin. Only about KeyCDN

You are right, KeyCDN is not a free service, but CloudFlare is free forever. But CloudFlare doesn’t support clear cache by tag, custom SSL and cache HTML page for free.

CloudFlare can make your images, CSS and JS file load faster, but not HTML, it doesn’t cache HTML file, but KeyCDN can. Both of them can help you prevent DDOS attack and let your server support HTTPS and HTTP/2, and CloudFlare is easier to use(however you have to change your DNS service).

Cloudflare does cache html. You can even force it with page rules.

Yes, use “Cache Everything” can cache HTML page, but I had tried this settings, it also cache 404 page. And CloudFlare doesn’t provide clear cache by tag feature for free so it’s hard to manage cache on a same domain.

I’m not sure what you mean?

The API certainly can clear by page (which is what your plugin does I suppose?) and tag?

https://www.cloudflare.com/docs/next/#zone-purge-individual-files-by-url-and-cache-tags

A quick heads-up. Only enterprise plan has “purge by cache tag” in Cloudflare. Ref: https://www.cloudflare.com/plans/pro/#compare-features