I haven't read much into the new series, but I'm gonna poke my head in anyway. Why would Bramblestar even elect Squirrelflight as his deputy? I'm unsure if they're still mates after TPOW and TFA, but wouldn't there be plenty of other warriors who didn't break the code that would be suitable? Other than Graystripe, who in reality is very old and left Thunderclan to join Riverclan in the first place proving that he's not an entirely loyal cat.
When I first read the books, I really loved Squirrelflight (although I loved Leafpool more), but reflecting back now I don't believe -- even with her loyalty and positive traits -- that she would make a good deputy, let alone a good leader.