If you were to add a use case that allowed for the removal of a "deleted" indication, Approach A would require that you read a tuple from the remove_tbl, delete that tuple, then use the data from the tuple to update the posts_tbl. Approach B requires a single delete.

If you were to decide on changing the wording of the "removed by" message, Approach A would require a moderate amount of work and data rewriting. Approach B requires a single change in code.

The Cons you list for Approach B are minor. Assuming the right index, the outer join to get a post and any remove data won't be an issue unless you're seriously high traffic.

I have trouble imagining circumstances where I'd even consider A as an option.