While F-15SA is the most advanced production model F-15 in existence, the first one only got delivered to the Saudis in February this year. We can rule it out as the plane that got shot down. That leaves the older F-15S and F-15C, and I am willing to bet good money it's a F-15S that was shot down. You don't see F-15C conduct ground strike missions normally as it's designed for air superiority. On second thought, it's the Saudis so who knows?
We know the Houthis have SAMs since many of them were former Yemeni military. The ones capable of taking out a F-15S are SA-2 and SA-3, both radar guided. They're older Soviet designs, outdated by today's standard. The radar warning receiver on the F-15S should have alerted the pilot to deploy countermeasure. If the pilot wasn't aware he was being targeted, it could be an infrared guided SAM. Those generally have very short range and low ceiling, so that F-15S would have to be flying pretty low and slow to get hit. Generally if you're above 10,000 feet, these types of missiles can't touch you. F-15E have been exposed to these threats before in other conflicts, and it came out unscathed.
Can't speculate on what happened as information is limited, but I suspect incompetence may be involved as Saudis are notorious for it. You can give them the F-35 and they might still end up getting shot down.
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