Until drop 6 I am able to Spfx web parts into sharepoint classic pages. I tried to create new spfx webpart using RC0 release and deployed through app catalog. I installed the spfx app into site collection and try to add the webpart into classic SharePoint pages the webpart was not displayed under webpart gallery.
Apparently some tenants didn't get the fix - can you check one last time?
Check it out this issue