The SPF Timeline shows the history of the SPF record if the domain was used as the Return-Path domain. Return-Path (also known as Envelope-From or Mail FROM) is used to specify the bounce address for email delivery failure notifications. This email address plays a key role in authentication since its domain is verified through SPF. Return-Path domains are extracted from the DMARC reports sent to GlockApps and shown in the SPF Domain column in the Details dashboard.
If there is no data in the SPF Timeline, it indicates that the domain has not been used as the Return-Path domain yet.