Using New Outlook on Windows for professional communication and productivity
Windows 10 will be out of support in October of this year, however, Classic Outlook will NOT be out of support until at least 2029, maybe even longer. There is currently no need to switch to Outlook New. As to the new installation on WIndows 11, there is no problem using a POP account in Outlook Classic on Windows 11. The "14-day automatic delete' is controlled by Outlook. Control Panel > Mail > Email Accounts, select your POP account and click Change. Click the More Settings button and select the Advanced tab. Here you will find an entry labeled Remove from server after (xx) days.
You will probably find this set to 14 days. You can change this to any number of days or turn it off by unchecking the box. This should restore you to manual Inbox maintenance, which you appear to have been using. As to getting Classic Outlook on the new computer, please see this Support article:
You can't open classic Outlook on a new Windows PC - Microsoft Support