It is only possible if you've logged these things with some tool or Splunk app that has measured availability in a way that meets your organization's definition of availability.
Perhaps you can look at the average time period between _internal or introspection log entries and see if some time period is beyond that by some number of standard deviations. However, this is not a guarantee of whether Splunk was available based on your organization's definition during the time periods where there are logs entries without indications of errors that may mean Splunk is not operating correctly, nor does it guarantee that Splunk was not available based on your organization's definition during any time periods that fall outside those number of standard deviations of silence/lack of logs.
Therefore, if you haven't gathered metrics to show operational efficacy of any service, including Splunk, it is extremely difficult, or simply impossible, to provide any availability reporting that would withstand any level of auditing or close scrutiny. I wouldn't bet an SLA on it without having a sound metrics gathering and reporting methodology in place from the start.
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