Does that mean it was available to the appraiser as well?
Yes, of course. That doesn't mean that connecting the dot between an existing septic and the setbacks to a creek is typical practice for SFR appraisers, though; even when there's a flood zone. I go to these lengths and more in my day job because I appraise a lot of land, of all types including SFR lots. I wouldn't normally do it for an SFR assignment though, not unless I saw something that raised my curiosity on it. Functional as of the date of the appraisal is just that - functional.
WRT what I do for a property with existing improvements and occupancy vs what I do when appraising land is that what's permissible for an existing structure is sometimes different than what's permissible for any new development that would be built on the site I'm appraising. Permissible when built is how most buyers and sellers treat such conditions, whereas the buyer for land has to consider the current development criteria.
Now I refer to what appraisers normally do, but IRL that also varies by locale. If there is a lot of flooding and well contamination and septic failures in an area the local appraisers will know to be looking for them. So what appraisers look for in one region might consist of different quirks than what they would look for in another region.
Remember, the County would have been the responsible party for issuing its permitting in the 1970s, even in TX. In lieu of information to the contrary most everyone would otherwise assume that a permitted system which functioned for 40+ years would still be sufficient for purpose.
I know, I know; you don't care about any of that. You just want to defer the point that FHA
may have declined the loan app if they were aware of the truth of these facts.
If I were a savvy atty I'd look into the sale and financing history of every parcel backing that creek (and others in the area) to see what FHA and other lenders have been doing with them. That would be more indicative (and factually supportable) of what they were actually doing than assuming they strictly adhered to that aspect of their MPRs in that region. Of which I have no idea one way or another. In appraisal parlance that's a question to be asked/answered, not an assumption to make.