Our city is no longer the shabby town __________ it used to be.
"That" can be filled in the blank, but why "which" can't be? Thanks!
Pope of the Dictionary.com Forum
It is not strictly ungrammatical, but highly unidiomatic to use 'which' or 'who' to introduce a restrictive relative clause predicated by a copular (as opposed to a simple transitive) verb, even in BrE, which often tolerates the latter.
Most native speakers would say
'Our city is no longer the shabby town it used to be'.
Our city is no longer the shabby town which it used to be.
is (on account of the copular predicator) unidiomatic, even by BrE standards, which would happily allow, e.g.
This is the money which I borrowed from you.
(Cf. AmE:...that I borrowed...).