I think if both parties agree to it, then why not?
I think if a writer is posting a unpaid collab, they really need to have a well thought post that explains everything. I think short projects may not be too many of an issue, but if they're asking for a webtoon original, then that becomes an issue. I think if you present yourself, as a writer, properly, then there is no harm in asking. But just be wary of the amount of time it takes an artist to actually create stuff. And, for the love of God, please don't put them on a schedule. That's my biggest pet peeve with unpaid collabs is having a schedule. This person is doing the work on their free will, do not have deadlines.
On an artist looking for a collab, it's more lenient. They are looking to do something fun, in their free time. They can pick and choose and look for a story to genuinely bring to life.
Then there is a third type of unpaid collaboration where the writer offers their services for proofreading and editing. I find this one to be very beneficial for both parties as having an editor and proofreader who has that creative spark to help reformat ideas is a great thing.
Ultimately, if you give a good, strong idea of what you're doing and don't say "HI, I'm a great writer. I have an idea for u. Please email me." And don't elaborate forward, yeah that's going to be an issue.