The "id" attribute was built-in on curves (point domain), mesh (point domain)
and instances, but not on e.g. point clouds. This mismatch causes issues because
of the special rules regarding built-in attribute propagation. Furthermore, an
implication of this was that the id attribute had to be an integer attribute on
a specific domain, which is not always ideal.
This patch turns the id attribute into a normal generic attribute, except that
some nodes internally modify this attribute in special ways. This may cause some
compatibility breakage in rare cases, but that can generally be easily fixed by
either removing the id attribute or setting it explicitly on the right domain. I
can't think if a feasible way to avoid this unfortunately.
The internal special cases for the id attributes are generally skipped unless
the attribute is an integer attribute on the point/instance domain.
Pull Request: https://projects.blender.org/blender/blender/pulls/146941
ae2034e6c5