This feature has been labeled as deprecated
. The recommended replacement is: shop=seafood
The reason is documented in Deprecated features
. You are still free to continue to use or interpret this tag as you see fit since OpenStreetMap does not have “banned features”.
Under no circumstances should you (semi-)automatically change “deprecated” tags to something else in the database on a large scale without conforming to the automated edits code of conduct
. Any such change will be reverted.
A voting process has shown that shop=seafood was preferred in 2010 by the participants for shops selling seafood, although the vote for this tag was rejected. See the discussion and the voting.