Did you look at the photo?

The crack is about 3/16" long and is in the very outside flange. It does appear to have been there for a very long time and there does not seem to be any propogation of said crack.

I would not be worried in the slightest unless there is extensive rust beneath the trailing arm that cannot be seen. Only in this case would I consider replacement.

I would wire brush the area throughly because there is some surface rust that is seriously developing there. Then I'd paint it with the POR-15 to prevent further corrosion.

There is virtually no chance of catastrophic failure from such a crack.