Don’t believe I’ve seen one of these before. Looks like some sort of bleed valve. Found it ‘Teed’ into a water heater inlet line today?
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Almost looks like a vacuum relief valve missing its cap.
https://manualzz.com/doc/59345942/cash-acme-22397lf-brass-1-2-in-mnpt-vacuum-relief-valve-m…
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Good research Ryan! 
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If I had to guess the water heater may have been installed in an attic or upper floor. That is the only place an atmospheric vacuum breaker would be needed.
Used when a pressurized vessel is located above the potable water piping.
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2018 IPC 504.2 Vacuum Relief Valve:
Bottom fed water heaters and bottom fed tanks connected to water heaters shall have a vacuum relief valve installed. The vacuum relief valve shall comply with ANSI Z21.22.
https://codes.iccsafe.org/content/IPC2018/chapter-5-water-heaters#IPC2018_Ch05_Sec504.2
Here is a UPC diagram
https://forms.iapmo.org/email_marketing/codespotlight/2018/june28.htm
Here is a good explanation of why these are used.
I’m seeing these a lot on new construction homes that use a heat pump water heater with a bottom feed cold inlet.
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All good information, Thanks guys. Curious thing is the water heater is top fed and located on the ground floor (slab on grade).
It is a requirement in some states, like Massachusetts, regardless of tank’s location.
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Could be a County Regulation. I don’t normally do inspections in this county.