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I'll cite SWOV.nl (Dutch institute for road safety research):

> The effect of (mandatory) bicycle helmets on bicycle use is not clear. Several international studies show that bicycle use decreases after the introduction of helmet laws, even though most studies do not find such an effect or only find a temporary effect [9] [42] [49]. [...]

> There are two international review studies of the effect of mandatory helmet use on the use of bicycles, both dating from 2018 [9] [42]. The first study [42] shows that the available research results are not unequivocal. It states that mandatory helmet use could indeed result in a decrease of the number of cyclists, but that this need not always be the case and that, if the number of cyclists initially decreases, that need not be of long duration. The second study is a mostly qualitative analysis of the available literature [9]. Based on their findings, the researchers conclude that there is little to no evidence of a substantial decrease of bicycle use due to the introduction of mandatory helmets. They have examined 23 studies/data sets and conclude that 2 of these studies support the hypothesis that mandatory helmets lead to a decrease of cycling, whereas 13 studies do not, and 8 studies show mixed results.

> The abovementioned review studies only concern research done abroad, in particular in Australia and North America.

https://swov.nl/en/fact-sheet/bicycle-helmets (Under "What is the effect of helmet use on the popularity of cycling?")