I reached out to them a couple years ago with this exact question and was told flat-out no. You might be able to sneak around it with an LLC but I think they also require you to have a public website for a plausibly banking-related business, which altogether seemed like too much effort to fake for what I wanted out of it.
So you don't have to be a business to use Plaid, but you do have to be a business to buy Plaid via the Sales channel rather than via the self-serve channel. Admittedly, when folks reach out to Sales and ask to buy Plaid and are told they're not eligible because they're not a business, this nuance is sometimes not communicated very well (or at all). We're working on it. :-)
In fact, we actually just this week launched a new sign-up flow to make it waaaay easier for non-businesses to use Plaid, so try checking it out -- after you go to dashboard.plaid.com and create an account, you should see a "Free trial" button show up on the homepage with a link to use the hobbyist onboarding flow.