The plugins pages on WordPress.com have been updated to include a link to download plugins listed in the WordPress.org directory. These are lists that are pulled directly from WordPress.org. Plugins are available for free on WordPress.org for self-hosted sites, but can only be used on WordPress.com with a paid subscription.
The text in the sidebar includes a link to an article explaining the difference between WordPress.org and WordPress.com, and this appears both when you are logged in and when you are not logged in:
This plugin is available for download and can be used on your self-hosted WordPress installation.
Themes hosted on WordPress.com also have a similar notification with a link to download the theme and use it on a self-hosted site.
This change is the result of concerns raised by developers that plugin pages on WordPress.com in some cases outperform those on WordPress.org in Google search results.
During that discussion, many developers were surprised to discover that their plugins created for WordPress.org were also listed on WordPress.com as only available with a paid subscription. In response, Patchstack has updated its readme file to ensure that WordPress.com users and visitors are informed that the plugin is available for free in the official WordPress plugin repository. This answer may now no longer be necessary, unless developers want to include a direct link to their plugins.
In a discussion on the Post Status Slack channel, some plugin developers said they would prefer a link to the actual plugin page where they can see and participate in reviews. The omission of a link back to WordPress.org may be intentional, as this would take users away from the .com site, which does not make it easy for users to upgrade to paid plans for using plugins.
Some developers also asked Automattic CEO Matt Mullenweg not to index those pages, but he responded that WordPress.com users should be able to search for listings on Google as well.
Developers have asked to know what percentage of their active installs come from WordPress.com versus WordPress.org or other hosting platforms. Mullenweg said there are currently no reports on this, but that the data could be interesting.
If people are providing greater distribution to unaltered plugins, I think that's great. I'm happy that all our plugins are duplicated and distributed by every host and site on the planet. – said Mullenweg during last week's discussion.