Focus Taiwan says it’s actually required not only recommended:
As these are also activities in which people may come into contact with strangers or may not be able to maintain social distancing, the booster shot requirement will also apply to visitors who are old enough to be vaccinated, it said.
As a result, anyone who goes to the gym, attends intercity religious events such as Dajia Mazu procession, and takes part in local tour groups arranged by travel agencies will be required to show proof of a third shot, the CECC said.