why remove ‘Purchased’

Thanks for your feedback.
We did not remove the purchased column for the WooCommerce order overview page.
WooCommerce removed their own column because of performance issues.

We made an improved version of this column in our WooCommerce integration add-on but it is only available in the pro version. Our improved version only loads when you click on the column, making the initial page load much faster than with the default purchased column.
https://www.admincolumns.com/woocommerce-add-on-for-version-3/

There is also free alternative available from developers who wrote the exact same column.
https://yithemes.com/blog/free-plugins/yith-purchased-column-for-woocommerce/