Days to sync
Mail Days to Sync option missing for IMAP email account on iOS 13
iPhone 11
Which email account are you referring to? Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other
Meanwhile look at the settings on Mail Days to sync
On the iPhone there is an option called "Mail Days To Sync." / "Remove After" This option allows you to save space on your iPhone by telling the server to stop pushing emails to your phone. There are several options to select: "No Limit" -- This option will send all the emails in your inbox and folders to your iPhone
Most mails will follow this sequence
Settings > Password & Accounts > iCloud > Mail (Scroll Down) > Advanced > Remove [Select from the options]
Settings > Password & Accounts > AOL > Account > Advanced > Remove [Select from the options]
Settings > Password & Accounts > Yahoo > Account > Advanced > Remove [Select from the options]
Except for Outlook
Settings > Password & Accounts > Outlook > Mail Days to Sync [Select from the options]
Watch the GIF below
Unfortunately Gmail does not have this option at all under iOS Mail app.
However that shouldn't bother you. because it hardly consumes space Settings > General > iPhone Storage > Mail App > [how much is it?] See the GIF below on my iPhone 6