After six years of not touching the registration page, I added "Login with Google" and "Login with Apple". (You can see exactly what that looks like here.)
Here are the results, after a week of testing:
The entire project took me around five hours, start to finish — I used django-allauth, as the prospect of relying on a third-party vendor for registration seems fairly terrifying.
Probably the single highest-ROI thing I've done this year (and I've done a lot of high-ROI work this year!) Couldn't recommend it enough.