You could easily oblige forwarding. You cannot shut someone out and then not allow them a) a backup of all data noe unavailable and b) and auto forward to an email of their choice.

Alas, email forwarding is fairly broken since DKIM/DMARC/SPF/SRS came into the picture.

As for not allowing people to save their data before shutting down, I agreee. That's a consumer rights issue though, and not a technical one.

Forwarding won't be enough if you need to reply from the same email address.

I assume a similar issue existed for telephone numbers. Was it wrong to require that they be portable?

When you dial a number, you generally look up the owner of the number range in a list, then initiate signalling contact with them. They may take the incoming messages, or they may point you towards another network the user can be reached on (i.e. if they have ported their number)

In essence, in the regulated telecoms space, the number block owner retains responsibility for routing to the new network after a number port.

And did all Telecom providers have the ability to do that sort of forwarding before the statutory requirement to allow numbers to be ported?

Legacy systems will be broken.