Well. The Nextcloud version is only just EOL. Only when there are relevant security updates and these are no longer available for Nextcloud 26 will tab.digital have to update them manually itself. But why they don’t just use a supported version, I don’t know. They don’t seem to use enterprise versions with longer support either.
You can create a new account at https://cloud.tab.digital and you get an account at https://kim.nl.tab.digital with Nextcloud 28. You can find also a list here.