You can search for the corresponding cmdlets, New-/Set-TransportRule. The UI is kinda limited in this regard, so best use PowerShell:
Search-UnifiedAuditLog -EndDate (Get-Date) -StartDate (Get-Date).AddDays(-90) -Operation "Set-TransportRule"
or use the Exchange Online admin audit log search:
Search-AdminAuditLog -EndDate (Get-Date) -StartDate (Get-Date).AddDays(-90) -Cmdlets Set-TransportRule