We cannot ever rightly say that general relativity is true, for it is a
scientific theory. A scientific theory is not something that can be
proved true, though it can be proved false.
In the hundred years or so since Einstein's introduction of general
relativity, no observation has produced data that would rule out general
relativity from its status as a candidate for the true description of
gravity. So in a loose sense, it still "holds true". But such wording
can be subtly confusing and, in my opinion, should be discouraged.
No experiment---not even any experiment related to quantum
mechanics---no experiment has exposed flaws in general relativity. It
is well known that general relativity and quantum mechanics are
incompatible. They cannot both be true. Still, no experiment yet
devised has been able to rule out either of those great theoretical
foundations of modern physics.
A theory of everything would need to be inconsistent, in some sense,
with either GR or with QM or with both.