Hello Maymai,
Thank you for using Apple Support Communities.
I understand that you want to delete all of the the Mail in the Trash at one time. The steps have changed from iOS 12, but you can still do it.
Go to the Trash folder. Select "Edit". Tap "Select All" and click "Delete". If you're following these steps and seeing "Archive" at the bottom right instead of "Delete", then you'll want to confirm the Trash mailbox behaviors. You can do this by going to Settings > Passwords & Accounts > select the account > tap the account name > Advanced > make sure that "Deleted Mailbox" is selected under the option for "Move Discarded Messages Into". After confirming this, see if you're able to delete all the trash messages at once.
Best regards.