Does Experience Cloud ID helps in identifying a user if a user deletes his cookie, if they visit next time will they be considered as new user or will be still be considered as old user?
It depends on the browser and which cookies they delete.
If the user deletes all cookies, yes, they will be considered a new visitor when they return after deleting their cookies.
If the user deletes just the AMCV cookie or all cookies on your domain things get a bit more complicated. In this scenario Firefox and Chrome would result in the same visitor whereas Safari would be a new visitor. This has to do with the way the 'MID' value within the AMCV cookie is calculated. If possible it will use a third-party demdex.net cookie to calculate the MID. In Safari this third-party cookie is blocked so it is unable to recalculate the same MID value.
I've put out a few videos about how the Experience Cloud ID Service (formerly Marketing Cloud ID Service) works on our community YouTube channel. You can view these videos here.