Marking the prospect plan as historical will prevent overdue steps from displaying the warning sign.

Outside of marking the prospect plan as historical, extending the 90 day limit or disabling the warning sign can only be accomplished via customization. However, there are a few Idea Portal entries to add these functionalities in a future service pack.

Allow change of how many days before warning on Prospect Plan Steps to fewer or more days as needed

Make the 90 day alert for last completed step configurable

Allow the warning on overdue prospect plan steps to be disabled