Will letsencrypt not sending email notification affect us? Should we need to do anything in ispconfig? Need advice.
If your ISPC server works as it should you've never received an email from LE. Though in case renewel might go wrong you will not get notified by LE anymore. So their suggestion to create an account at Redsift (I don't know if there might be others) isn't a crazy idea. You'll have more insurance to spot renewal problems before a certificate is actually expired.
I have received e-mails from LE. If I removed a website but forgot to cetbot --delete the certificate files, LE eventually sent e-mail about the certificate going old.
Btw. there is a big improvement for this in ISPConfig 3.3. which can now remove the certs automatically and also clean up old certs.