Twilight never told anyone in full scale how much damage Starlight did to the ponies of Our Town (no one besides the mane 6 and other ponies know it) and she also never told anyone how much shit Starlight did to the timelines (we can assume she told the mane 6 in the season 5 finale but never in a full extent) so only Spike, Starlight and Twilight know the full truth.

Do you agree with Twilight here about telling no one? I sure do, from a princess of friendship, that was the more friendly gesture after season 3, not ruining starlight's life when she could.