Does Buttondown offer A/B testing?

Not at this time, no, though you do have two options to approximate A/B testing:

• Tag-based A/B testing: You can create two separate tags and send two different emails to each tag, and then compare the difference in open rates, click rates, etc.
• random_number merge tag: You can use the random_number merge tag to randomly assign a percentage of your subscribers to one of two groups, and then send two different emails to each group. Check out the random_number documentation for more information.