There is a new dasboard called Sources, that replaced Sources tab.
Graylog 3.2 changelog also contains this information:
Sources page has been removed and converted into a dashboard.
This is a list of all sources that sent in messages to Graylog.
This is a list of all sources that sent in messages to Graylog. You can narrow the timerange by zooming in on the message histogram, or you can increase the time range by specifying a broader one in the controls at the top. You can also specify filters to limit the results you are seeing. You can also add additional widgets to this dashboard, or adapt the appearance of existing widgets to suit your needs.