No need "it" before was;
The S is followed by adj. cl.
The poor tabby----S
(that) she got----Nominative, adj cl.
was very meek and mild-----vt +o------adv cl of manner, of result, of comparison, of condition -------No need "it" before "was" because the S being "The poor tabby", not "it".
through all the bad treatment-----adj phr-----of description, of demonstration, of relation , of result.
Rewritten with the same meaning with a noun cl:-eg:-
She (the poor tabby) was----S+vt
very meek&mild----adj phr
through all the bad treatment---prep adj phr
(that she got=that she gotten=that was gotten in her)----noun cl as o