open drain pipe in crawl space

This was added to the drain system to accept waste from a water filtration system. Is there an exception somewhere that would allow an open drain pipe in the crawl?

No. That should be capped.
Unless there is a p-trap below, and the open fitting is acting as an air gap.

This was added to the drain system to accept waste from a water filtration system. Is there an exception somewhere that would allow an open drain pipe in the crawl?

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There are several things that must happen with this drain.
1. it must have a trap to prevent sewer gas from entering the space. This gas will present an explosion hazard. It may also present a poisoning hazard. Hydrogen sulfide gas is very toxic. While in low concentrations it smells like rotten eggs, at toxic levels it paralyzes the sense of smell and then kills.
2 the drain needs an air gap. This means that the end of the drain pipe from the equipment must be above the edge of the pipe. That is so that if sewage backs up the drain, it will not go into the equipment. (It also will present a way to get sewage into the crawl space. The current installation will also allow sewage into your crawl space. Further, it does not appear to have been installed by a professional. There is no visible glue on the joint.)
If the equipment drain is for filter backwash, it can be plumbed into a sanitary tub with the washing machine drain or into the same drain as the washer is.

Terry, Your answer is much better than mine, but I love talking about the reasons behind the rules. ( and possible optional solutions)

It is always a bad idea to have an open drain of any kind below the flood level of the system's fixtures, especially in a crawl space where the problem might not be discovered for a considerable time, because that will be the first place a backflow will come out.