To make things concrete, we've two MySQL tables (simplified) as follows:

posts_tbl
post_id author_id message deleted_flag

removed_tbl
post_id message deleted_by_id date

Let's say we have too approaches A and B.

When a message that is deemed inappropriate is removed via a button click, Approach A goes likes this:

In posts_tbl, the "message" field is updated with the replacement text "Removed by USERNAME" and the "deleted_flag" column with a value of 1 (the default is 0) (Note: the username is available at the point of update, so it's updated into the message filed as part of the replacement text).

In removed_tbl, the original message is inserted into the message field, together with the post_id, the date, the member id of the person carrying out the deletion.

Approach A's points are:

a) When displaying posts in a web page, all posts can be directly retrieved from posts_tbl, including messages that contain "Removed by USERNAME".
b) Saving of processing time because there's no need for additional code to check whether a message was removed.
c) Greater flexibility because different pieces of data are stored in two different tables.
d) Greater expandability. When a different reason for removing the post is needed, a new column can be added to removed_tbl to store that reason.

The sql to retrieve messages from posts_tbl will be straightforward without having to link to removed_tbl to retrieve the username of the person who deleted the post (assuming the other information in removed_tbl is not needed for display). The code will be simpler too.

Approach B is:

removed_tbl should be changed to:
post_id deleted_by_id date

When a removal is carried out:

In posts_tbl, the "message" field remains unchanged while the "deleted_flag" column is updated with a value of 1.

In removed_tbl, the necessary information is inserted (post_id, member id of the person deleting the message, date)

Approach's B points are:

a) The original message should not be touched in posts_tbl.
b) There's duplication of data if the message field of posts_tbl is updated with the string "Removed by ...".
c) There's also duplication of data because "Removed by ..." and "deleted_flag" both indicate the particular post has been removed.
d) The string "Removed by ..." should be added in the code for outputting to a webpage, so that it doesn't get stored anywhere. When a different wording is needed e.g. changing "Removed by ..." to "Deleted by ...", only that the hard-coded string needs to be changed.
e) posts_tbl stores all original posts, so it's easier for retrieval.

Cons of Approach B:

a) The sql to retrieve messages will require an additional condition to link posts_tbl to removed_tbl to retrieve the username of whoever deleted the post.
b) The code will need to check for whether a post has been removed so it can output the string "Removed by ...".