Emails

You have an iPhone XS, and you seek messages from older than March 2020. To determine whether these messages still exist on the mail server, you will want to log into the provider’s web mail interface. This interface is available from most mail providers. This web mail interface may allow you to find your mail messages, and may particularly allow you to find mail messages prior to March 2020, as you are seeking mail messages from prior to March 2020. If the messages from prior to March 2020 are not available using the web mail interface, then you will not find those messages on the mail server using your iPhone XS.

Yes, I understand. As MrHoffman correctly suggests, you need to check the web version of your mail account to see if the messages can be seen past the start of March. If they can't be seen, they are likely gone. If they can be seen, you'll need to contact the provider of your mail account to see why you can't see them on your iPhone. Not all email accounts I have offer Mail Days to Sync.

Go to Settings > Passwords And Accounts > Click the mailbox and see if there is a setting for Mail Days to Sync - Adjust as appropriate there

Are the messages still available within the webmail interface available from many mail providers?