Treasury has published a paper (by two MOTU economists) looking at the impact of 90 day trials periods. The findings are:
We find no evidence that the policy affected the number of hires by firms on average, either overall or into employment that lasted beyond the trial period. We also do not find an effect on hiring of disadvantaged jobseekers. However, our results suggest that the policy increased hiring in industries with high use of trial periods by 10.3 percent.
So the impact was in industries that were using the trial periods. Makes sense. Many industries won’t want to use trial periods, but those that do did see an increase in hiring.
Specifically they found:
we estimate the policy effect to be a statistically and economically insignificant 0.8 percent increase in hiring on average across all industries. However, within the construction and wholesale trade industries, which report high use of trial periods, we estimate a weakly significant 10.3 percent increase in hiring as a result of the policy.