After some theorizing and putting my tin foil hat, I think this was the original story/plans of SOL:
WINTER
Based from this user called MisterZone, it seems like Saeko was originally going to be the alternate POV chapter for the Winter Episode. As for how and what content would be here, it's most likely NTR with her pet dog Fuku during the Christmas Holiday Trip depending on your choices. I say depending because the texts in the game contradict the validity if she does indeed have THAT kind of relationship with her dog but I think I figured it out.
Saeko will get a platonic relationship with her dog at the end of the game if Tsugumi gets corrupted by the teacher. Why? Mostly because of the constant conflicts Saeko and Tsugumi will have due their strained relationship from the hooking up Tsugumi is doing with the teacher as well as all the smoking at their place. This prompts Saeko going to her Christmas trip on bad terms with Tsugumi. However, on the Tsugumi gets with Satoshi path, Tsugumi stays with Satoshi and Saeko and Fuku leave for the trip on better terms which I think would allow the NTR to actually happen.
In other words, Saeko and Fuku will always go on a Christmas trip but the difference is in one path, she gets laid by the dog, the other she just starts to hate Tsugumi even more because of the teacher and that prevents her from getting banged by her dog I guess? Plus, the game is called seasons of loss, so in the alternate path or good ending of Satoshi where in he ends up with Tsugumi, something is still lost because her sister gets ntr'ed by the dog.
As for why it was scrapped, my best two guesses is either it was such a niche scenario that NTRMAN didn't bother to invest time art and money for it since, like I said, it caters to a niche crowd. The other reasons is like NTRMAN has done before, lost passion in the storyline and just gave up on it.
SPRING
We gotta remember that the last teaser for SOL was all the way back in late 2022 and teased this gyaru character that unfortunately never appeared but her design I think was reused for the character of Chihiro:
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by late 2024 and early 2025, the newer characters of saki and the rest of the new comers for SOL were already being developed with the original plans at this point gone and scrapped as NTRMAN made AOTG and TOTD.
As for what would've happened in Spring, considering that satoshi becomes a fuckboy later down in the ntrman timeline, my guess is that the gyaru girl would've helped satoshi become more fit, be more bull'ish and get tanned skin as well as be satoshi's love interest and someone who teached him how to have "game" or how to basically be flirty and more confident. Despite that, this is still an NTR game so Satoshi will somehow loss the gyaru girl in his pursuit to become future satoshi. As for how? IDK. Best guess is she gets blacked by some dude or something but in this version, we would've seen how Satoshi' became more of an asshole, dick and bull-ish because of the gyaru
SUMMER
This is all speculative since I doubt NTRMAN ever got this far in his original concepts for SOL but Summer might be Satoshi's first victim of NTR-ing considering he himself is fully corrupted by then and his innocence finally lost at the end of the season.
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As for why these were scrapped, my best guess is that the focus of NTR was gone and instead replaced by trying to explain how Satoshi became who he is. TLDR, he was writing an origin story with NTR rather than NTR stories so maybe that's why. Could also be NTRMAN not liking his originals plans or worse, his writing just got shit over time. Who Knows?