At times, we may need the web part maintenance page to remove an erroneous web part from a page in SharePoint. In SharePoint On-premises and in Classic SharePoint Online, we used to append ?Contents=1 to the URL to open a page in web part maintenance mode that offers close/reset/remove functionalities on the selected web parts.
Now, in Modern SharePoint Online, The web part maintenance page can be obtained by appending ?maintenancemode=true to the URL! E.g. https://crescent.sharepoint.com/sites/Marketing/SitePages/Home.aspx?maintenancemode=true
Unlike the classic web part maintenance mode, The modern SharePoint Online web part maintenance mode displays summary, manifest, and data tabs with information about each web part which helps to debug and troubleshoot. If you want to delete any web part, You can edit the page and remove the web part, and then republish the page.