deno.land/x is a hosting service for Deno scripts. It caches releases of open-source modules stored on GitHub and serves them at an easy-to-remember domain.
The basic format of code URLs is
https://deno.land/x/IDENTIFIER@VERSION/FILE_PATH. If you leave out the version it will be defaulted to the most recent version released for the module.
deno.land/x warns you when you are implicitly importing the latest version of a module (when you do not explicitly specify a version). This is because it can be unsafe to not tag dependencies. To get rid of the warning, explicitly specify a version.
Module versions are persistent and immutable. It is thus not possible to edit or delete a module (or version), to prevent breaking programs that rely on this module. Modules may be removed if there is a legal reason to do (for example copyright infringement).