Others left too, most notably Count Dooku.
While this is true, he leaves it over a decade before Order 66, and then goes on to become a Sith, whereas Ahsoka leaves within a year of the end and for more “correct” reasons. The audience is on Ahsoka’s side but not Dooku’s.
I don’t remember her fighting Sidious.
In Rebels. There’s no clash of sabers but she holds him out of the WBW for a few moments.
Lots of people did.
Yes, but not many Jedi did (despite what Mace Windu may insist, Ahsoka doesn’t quite count as an ordinary civilian). We’re led to believe in the OT that Luke is literally the Last Jedi - the fact that Ahsoka has rejected the actual Jedi Order by that point so she doesn’t count is a very smart bit of weaving between the lines of the pre-existing text. “Luke, when gone I am, last of the Jedi will you be. Your… father’s apprentice is still alive, but count not, does she, because white her lightsabers are, so bother to mention her ever, I will not.”
“No… there is another.”
“Oh, Ahsoka? Yeah I suppose I probably could make a ghostly appearance to her and try to get her back on side -”
"No, Obi-Wan! A foolish idea, that is. I mean Leia - who, convincing Luke to let die, we were, just a moment ago… "
Obviously I slightly exaggerated Ahsoka’s unlikely list of accomplishments for my previous post, because it was essentially just a set-up for the punchline that I love seeing her character despite knowing she’s overused. NFB makes some great points on how Ahsoka could be made more interesting in the post-OT era - I can only imagine the YouTube meltdown about how dIsNeY rUiNeD aHsOkA if they did place her at odds with Luke Skywalker, though.