this CDN was made on glitch, it serves the user with the requested content through express, it is a project with minified versions of different frameworks such as socket.io, bootstrap (JS and CSS), jquery and sweetalert. These are all the latest versions and is completely free.
Yes, and the links on repl.it send you to cdn.ptuk.tk so it uses the Glitch version so you need to edit the links so they just go to e.g. /bootstrapcss instead of https://cdn.ptuk.tk/bootstrapcss
I agree with @ihack2712, its just a lot better and faster to use professional cdns like cloudflare, also the chances of cloudflare going down is very low