Another problem with email address as user ID is that much of the public (most I'd guess) does not have a permanent email address.
Many use an email address provided by their ISP. What happens when they move out of that ISP's territory? Or, if they are someplace served by multiple decent ISPs decide to switch providers?
Many use addresses from gmail, outlook, yahoo, and similar. Those at least keep working if they move, but still have some risk. If you use multiple services from the companies that own those and do something to get banned from one of those company's services that might also get you banned from their email service.
Best if a site insists you use email as user ID is to use an email at a domain of your own. That won't be free because you'll have to rent the domain, and pay someone to handle your email (most people will not be up to running their own email server), but if the domain is at one of the long established TLDs and you don't do anything too illegal and it isn't close enough to the name of an established company that you could lose it over trademarks you can probably keep it for the rest of your life.
Whoever you use to actually handle you mail might go away or kick you off, but as long as you still have the domain you can switch to some other mail handler and point the domain's mail records in DNS to that new handler.
If you want to be sure that there is no risk of being accused of being a domain squatter or losing the domain in a trademark dispute pick a name that will not be at all similar to any business name or famous person name. I've got my ham radio callsign as a domain under the US TLD for example.
If you aren't using your own domain, at least check with any important site that you use that requires email as user ID to make sure they have a way to change the email so that if you do end up losing your current email you can update the site. That might not work if you lose the email without warning, but at least it can help in cases where you know you are going to lose the email such as switching to a new ISP.
It might also be a good idea to keep a list of all sites you are using where you will need to change the email as user ID if you are going to move, so fixing it can be part of your moving checklist.
In the US both of the login servers that more and more government agencies require you to use for online access, ID.me and Login.gov, use email as user ID. Both allow you to change that email (add the new email as a secondary email on the account, then change the new email to be the default email). It would be really annoying to not remember to do so until after you have lost the old email, and so find yourself unable to login to your IRS account or your Social Security account.