Sometimes characters leave without us ever realising our last goodbye was the final goodbye. They come on our screens and then they disappear for a bit only to reappear and then poof, gone. You think where have they gone, but details are nowhere to be found as if they never existed. Life moves on and its not important anymore. then out of the blue a comment is made about them and you're left with more questions than answers.
So Let's answer the question.
When did Tara and Willow say their final goodbye to Miss KittyFantasico?
The Yoko Factor - They went to the pound but brought nothing home, its all a delusion!
Family - Its the last screen appearance, maybe Mr Maclay got hungry waiting in Tara's dorm room?
Forever - We are led to believe Dawn, a crossbow and Miss Kitty had an altercation, could grief have been the trigger?
Tough Love - Willow was already dedicating her future to look after Tara, could a cat have been a responsibility too far?
Spiral - Unattended pet while off on a road trip, maybe the sneaky kitty finally got found by the RA?
Summer between series 5-6 - The Summers house doesn't seem the best suited for pets. Perhaps Miss Kitty was working abroad to send money home for the heavily disputed rent money?
Flooded - Miss Kitty got no credit for paying the bills and trying to tackle some of the debt so unlike her checks, she bounced.
Life Serial - She lost big at poker night.
Once More With Feeling - Miss Kitty's rap about geopolitics of the mid eighteenth century was cut and so she cut ties.
Tabula Rasa - She forgot to show up again.
Smashed - Miss Kitty doesn't do broken homes.
Other - You decide, please say in comments.
Summer between series 5-6, it makes the most sense if Dawn was involved. Plus with the added responsibilities for everyone and changing living arrangements, its likely Miss Kitty couldn't adjust so had to find a new home where people could spend time with her, instead of having owners in and out at all hours while in an emotionally turbulent household. Not to mention all the brawls that happen in the house, I don't think either would want Miss Kitty in an unsafe environment.