I think, this is more a question towards Nextcloud GmbH, but I don’t know where else to ask:
I made a Webextension that supports Thunderbird 68+. It’s (loosely) based on the old version and so far contains the same icons. Is it OK to use those icons? Who can I ask directly?
You are right about the copyright, you should ask Nextcloud before using it. However, I think the best way would be to revive the old app and give you access to the code that in future there is one more maintainer.
The right person to get in touch with is @jospoortvliet.