Hi Katie!
There’s not a ton in the literature about 3/4 footplates. 3/4 footplates are likely more flexible and therefore allow more ankle dorsiflexion during swing phase.
I did find one article indicating that AFOs with full-length footplates produce increased stance phase PF moments compared to no AFOs (wherease 3/4 footplates were no different than no AFOs). That same study found that AFOs with 3/4 footplates resulted in significantly less than normal dorsiflexion in late stance. It was hypothesized that AFOs with full-length footplates increased DF ROM and PF moment in late stance by delaying or impeding heel rise more than the 3/4 footplate. (https://www.sciencedirect.com/science/article/pii/S0003999309001221?via%3Dihub)
It seems that they may increase walking speed. In another study, almost all patients with 3/4 footplates showed an increase in the walking speed, although they were not statistically significant. Meanwhile, the ability of body to move on weight bearing limb and fast weight distribution is the important factor affecting the walking speed. It seems that the 3/4 AFO allowed more dorsiflexion in mid foot; and the third rocker was fulfilled easier during the terminal stance phase. (https://pdfs.semanticscholar.org/4ef1/5e8e617949a7f88e73f017491fad6f553af6.pdf)