I have been using https://simpleicons.org for a while now. It is super light and looks great. License is CC0. Although I prefer inlining the icons over using <img> tags.
The point of CC0 is that in countries (like the US) with the concept of releasing works into the public domain—relinquishing all copyright claims on the work, that's what you are doing with CC0.
In countries where you can't legally release works into the public domain, CC0 is a license that tries to do as much as it can through a license in those countries.
As I understand it, "code licenses" like MIT and "content licenses" like Creative Commons have slightly different intended areas of application, and you shouldn't mix them. E.g. "linking" means different things for code and images.
So you'd use e.g. CC0 for an icon set together with MIT license for some javascript/whatnot code that goes with your icons.
CC0 can be used for source code. For example, the FSF lists it as a GPL-compatible free software license and states: "If you want to release your work to the public domain, we recommend you use CC0."
CC0 reverts to "Public Domain" in countries/jurisdictions where this is a thing. When this is not possibly it tries to legally define the closest thing possible.
"Public Domain" in particular means that the author distances themselves entirely from the work and all the rights pertaining to it. You don't need to credit them, I believe you could even claim it as your own (that'd be ethically dishonest IMHO, but legally fine). Second, the author also distances themselves from any responsibilities regarding this work (such as accuracy, fitness for any purpose or safety).
From what I've learned about copyright and IP law (extracurricular course next to my CS education, highly recommend if you get the opportunity) and reading, it's especially these last two points that are sometimes (in some jurisdictions/countries) difficult to get rid of or distance yourself from, as an creator. They don't always allow it. For instance (someone correct me if I remember this wrong), in the Netherlands a creator cannot fully distance themselves from the right to claim "I created this". Meaning that ghost-writing is contractually not possible and relies on the honour of the ghost-writer to keep their mouth shut. It's a bit of a vague thing and we didn't delve too deeply into that example. It's generally a minor thing.
Same goes for whether a creator can ever fully distance themselves from any responsibilities over the work they created. I can imagine some jurisdictions may not allow that either. I'm not sure how that works in NL (was not covered in the course).
Anyway in this sense, CC0 is a lot like the WTFPL without the profanity, I guess :)
Just last night I was playing with Primitive [0], which tries to reproduce photographic images by building up layers of shapes. I found this via SQIP [1], was discussed on HN last week.
I haven't tried this with line art like icons or logos. I usually just hand-optimize simple ones, then run them through svgo [2].
