In reply to Gobinda.

Probably the best ad for a pro version is a clear statement in the plugin's description showing the differences between the free and pro version. I look for this before installing plugins and am surprised and frustrated when I can't find it.

Another good way to promote the pro version is to display pro features on the plugin's settings page with clear, unobtrusive labels (like "(Pro)"). However, a settings page where most of the items are labeled "Pro" would also be unacceptable. Some plugins with many pro-only features put them on a tab labelled "Pro" or group the free features together.

Also fine is to display a reminder of pro features in an unobtrusive sidebar.

A bit less acceptable is sending email to the admin promoting an upgrade.

Automatic plugin updates have removed an important opportunity for promoting the pro version. Manual updates had provided a great opportunity to remind us of the plug-in's features, especially improvements in its pro features. Perhaps automatic updates should be reconsidered?

The types of notices Clarity seeks to block are not acceptable at all. Also unacceptable are notices to upgrade to pro triggered when undisclosed limitations are reached.