I appreciate that at least they stayed consistent with the Sword of the Creator being Crest of Flames-exclusive and, as such, Byleth can no longer use it post-CF and is shown using a normal sword instead in their solo and S-Supported Jeritza ending pictures; makes it all the more baffling that they're still alive despite their heart being destroyed and that the Sothis S-Support exists in that route, but i'll give them credit where it's due for keeping the SOTC's lore consistent.

Post Tru Piss, there's no Crest Stone left for the SoC!

So I'd say it's not that it's CoF exclusice, but rather something like without a crest stone, a relic, even the SoC, doesn't "work" anymore.

Adding insult to the injury though, Post!Tru Piss Billy uses Rhea's sword, aka, a Holy Weapon that could be used by everyone (human and nabatean!) and even heal non-crested people : Billy uses at the end of the "we will make a world for humans!" route a sword that conveyed coexistence between humans and nabateans...

Sothis' S-support in this route really felt odd, because she acknowledges Rhea at least, but has no words for her passing when Rhea died crying for her.

Some people tried to theorise that this Sothis isn't the one Rhea remembers so she wouldn't have feelings about her... but idk, it still feels dead wrong, especially since Sothis remembered feelings of joy and sadness in Zanado, and ultimately remembers how Rhea is a her kid thanks to the lullaby (and in SS when she "talks" to her).

If Sothis was a better written character and not accidently written to be the most toxic parent in the FE series, I'd maybe write something about her feeling so bad that, again, she was used to slaughter one of her own children (Nemesis first in Zanado, and now Billy in Tailtean) but as canon!Sothis is, I'm not really motivated to do something like this.