Yes, it's really free for adults. You may get emails inviting you to donate but, if my experience is anything to go by, you won't be pressured to donate, or be bombarded with emails.

Hi positivelyhumanistic,

Yes, Khan Academy is a non-profit organization and provides course content for free for all learners!

You will occasionally see site banners and receive emails about donations, but the banners can be dismissed and you can turn of emails via your account settings.

Khanmigo is a paid offering available to all ages of learners in the U.S., but it is not a required feature and your learning experience or access to the course content will not be restricted without it.

I hope this helps! Please let me know if you have any further questions!