Hi,
No, the legacy settings are supposed to work with PHP 7.3. It's just that they are less tested than the non legacy ones, since the vast majority of people don't use them anymore as they are old stuff.
The main reason you could look at disabling them would be to use the new interfaces which work better and have more features.
For example, after enable the checkout legacy, if you look at the "checkout" tab of the HikaShop configuration, you'll see the new checkout workflow editor which has much more options than the old one.