The panel is loaded but not to capacity. To bring the current sub-panel up to the imminent adoption of the 2017 code, half-sized AFCI or combination breakers would be great.
Is there such a thing as THQP1120AF or THQP1120AF2?
(15A versions too)
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I found GE’s residential circuitbreaker page. Given that it’s correct and complete, GE doesn’t sell a 1/2” QP line AFCI or AFCI/GFCI combo breaker. I think they would be good sellers.
When a breaker fits two where one goes, we call that type of breaker a double stuff.
GFCI and AFCI breakers don't come in double stuff. The electronics will not physically fit.
Fortunately for you, anything that was legal at the time it was installed is grandfathered. You don't need to modernize your wiring everytime Code is updated.
If you must comply with current Codes, you either add a short stick of EMT conduit to a 4-11/16" square box with 2-gang mud ring, and fit 1-2 AFCI receptacles or deadfronts there... Or you replace that panel with one that has enough spaces to not depend on double-stuffs.
This is why people here stump so hard for readers to buy rather large panels, and recommend 40-42 space for any house. Thee incremental cost (at panel purchase time) is a couple of pizzas, your situation is expensive.