Maybe I am understanding you wrong, but the all point of copyleft is to allow people to share your work on other platforms.
You can use the licence you like, with the accesses you fancy for you audience.
Libre culture is all about empowering people. While the general philosophy stems greatly from the free software movement, libre culture is much broader and encompasses other aspects of culture such as music, movies, food, technology, etc.
Some beliefs include but aren't limited to:
Check out this link for more.
I've looked into the ways other forums handle rules, and I've distilled their policies down into two simple ideas.
Please show common courtesy: Let's make this community one that people want to be a part of.
Please keep posts generally on topic
No NSFW content
When sharing a Libre project, please include the name of its license in the title. For example: “Project name and summary (GPL-3.0)”
Libre culture is a very very broad topic, and while it's perfectly okay for a conversation to stray, I do ask that we keep things generally on topic.
Community icon is from Wikimedia Commons and is public domain.
Maybe I am understanding you wrong, but the all point of copyleft is to allow people to share your work on other platforms.
You can use the licence you like, with the accesses you fancy for you audience.
Copyleft licenses require that derivative works maintain that same license. Not all the Creative Commons licenses are copyleft. CC-BY only requires that attribution be given in the derivative work. That's the one YouTube offers as an option for uploads.
If you want your video to be copylefted, you should use one of the SA (share-alike) licenses. See https://creativecommons.org/share-your-work/cclicenses/
Yes I know.
As I say using copyleft work on other platforms is doable. On YouTube it's doable if you make a derivative work, but on peertube ou dogmazic.net for instance it is perfectly fine.
But as I say maybe I understood wrongly the initial question...
I'd say to ignore the platform licensing and just make sure that the license appears in the media itself (which it should, anyway, in case anybody finds it randomly) and marked in descriptions.
YouTube seems interesting, because there's so much garbage listed as CC-BY that almost certainly doesn't have any legitimate permission for it, and I've never found actual Creative Commons content through that route, so that probably informs my "just ignore it" thinking...