Is it safe to use IPad that cannot upgrade IOS any longer?

Due to the system architecture of iOS/iPadOS, unless jailbroken, your iPad is not susceptible to traditional malware infection per-se. However, as with all computer systems, there are still vulnerabilities and exploits to which you remain vulnerable. For older devices, no longer benefiting from regular security updates, the risk of an unpatched vulnerability being exploited significantly increases. 

Given that your device is well within its twilight period of hardware support - and no longer receives system software updates - if used online (and especially if used to access financial or e-commerce Apps or services) you would be well advised to consider replacement with a more recent device - in so doing benefiting from both enhanced functionality, regular security updates and full update support.

Quite safe, really. It's highly unlikely that your iPhone could be compromised just by an email, even in a relatively old version of iOS. Of course you always want to practice "safe computing". See:

Effective defenses against malware and ot… - Apple Community

Regards.