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Still accessible via the web, of course (and from what I understand Parler is a PWA that can be installed to the homescreen).

Until their hosting decides to block them, or my ISP, or several AS’s in the middle decide to block them and there’s no route to them.

The answers, respectively, would be: start your own hosting provider, start your own ISP, deploy your own Internet.

Just to be clear: I’ve never used Parler.