Does iCloud backup my email on my iPad Pro even if I accidentally deleted my past emails?

Apple provides a support document called What does iCloud back up? which answers your question:

Here’s what iCloud Backup includes

• App data
• Apple Watch backups1
• Device settings
• Home screen and app organization
• iMessage, text (SMS), and MMS messages2
• Photos and videos on your iPhone, iPad, and iPod touch2
• Purchase history from Apple services, like your music, movies, TV shows, apps, and books3
• Ringtones
• Visual Voicemail password (requires the SIM card that was in use during backup)

Does iCloud backup my email 

See Apple's support document Delete email in Mail on iCloud.com

iCloud is the source of your email, not a tributary. It is the centre of a syncing service for all your @icloud.com email addresses. If you are signed in to iCloud and delete an iCloud email on a device it also deletes it from all your devices and iCloud.

However, the email will be moved to a bin folder you have set - typically in the 'On My Mac' bin mail box - and be erased after a period you have set in your Mail app preferences (Mail > Preferences > Accounts > [iCloud Mail] > Mailbox behaviours)

Note this may not apply to any email you receive from other service providers - such as your ISP or an internet service like Google. Each email service has its own setting in Mail preferences.

No. iCloud backup does not include a backup of your email - as your Mail provider’s mail server maintains a working copy of your sent and received email messages. If mail messages are deleted, your mail provider may keep deleted mail for a period of time prior to performing permanent deletion.

More information about iCloud backup can be found here:

What does iCloud back up? - Apple Support