cPanel plug-in?

Considering the number of people running Nextcloud in a shared-hosting environment without shell access to perform command-line maintenance, has anyone heard of (or has Nextcloud themselves considered) creating a cPanel plug-in offer this sort of administrative maintenance?

Cpanel does offer SSH access, which you must enable as described here. It is simply off by default. Not sure on who your hosting is, but the button looks like this:
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File Manager is another way to access the entire filesystem for maintenance.

There is also a console interface, but I do not recommend it when you can use SSH instead. Totally Insecure, but it also exists at:

Mail account support is available from Cpanel Mail Sync app

I know cPanel can offer shell (SSH) access, but often this is disabled by the provider.

A shared hosting provider that offers Nextcloud but who disables shell access would likely still offer a Nextcloud-specific cPanel plug-in.

Nextcloud will not have control over your Cpanel by design. Cpanel itself is closed source. In addition, the provider has ultimate control.

If your provider does not do what you want, your best bet is to ditch that provider. It is basically a stuck position if you choose to use a provider who will not do what you need. Hope this helps.