MySQL - Advice for using two tables, one each for open and closed records?
Add a column to distinguish open/closed.  It should be small (TINYINT or ENUM).

Then pick one:

Plan A:

Then have most indexes be composite and start with that column.  These indexes will make it efficient to find just opened (or just closed) rows.

If you also need to find both opened and closed, then also have index(es) that do not start with the new flag.

Plan B:

PARTITION by the new column.