Postgresql upgrade generated vaccumdb script not vacuuming?
That's not a VACUUM script, it is an ANALYZE script. That's why it isn't vacuuming. The confusion arises because vacuumdb can also be used for ANALYZEing a database.

After pg_upgrade you have no table statistics at all, so most queries will perform very badly. That's why it is essential to mend that as soon as possible.

Since pg_upgrade copies (or links) the table files, there will be as many dead tuples after the upgrade as there were before. That is not alarming, and autovacuum will take care of it in due course.