As time goes by, we sometimes find it's good to have rules written down for things relating to how we operate. This is where we write these rules. Please don't change them without prior discussion.
It's pretty simple: if a plugin is, or will shortly be (eg. author giving heads-up about imminent release), publicly available for download, it is eligible for addition to the appropriate masterlist.
Plugins that are not available to the public are not suitable for inclusion in the masterlist - if someone reports such a plugin, tell them that it is more appropriate for them to add any necessary metadata using a userlist rule. Such cases include private beta plugins, or more generally plugins distributed amongst a select group of people, or plugins for which you have to pay to access (which is against Bethesda's rules, but has undoubtedly happened before).
Sometimes it is necessary for a maintainer to download a plugin to investigate it, eg. to double-check information provided by a user. In such cases, we generally ask the user to provide a link to the plugin in question. In the case of plugins not yet publicly available, we may ask for a link to be provided privately, e.g. by private message.