It's the protocols on top that have the vulnerabilities, not the frequencies. For example, Wi-Fi 6 will again use the 2.4GHz band, but that doesn't make it less secure than Wi-Fi 6 on 5GHz. That said, 802.11ac Wave 2 only uses 5GHz, so looking at it that way, there is a relation between the technology used and the band it's using.
Another aspect is that the 2.4GHz band is more crowded, so it may be more prone to indirect attacks. For example, let's say you have a network on the 2.4GHz band called "MyCorp". An attacker may count on the fact that users experience issues on the 2.4GHz band, and create a rogue SSID called "MyCorp-5G" tricking your users to connect to that instead. Just an example, I'm sure there are many more.
2.4GHz also reaches farther, so from that point of view the attack surface is bigger.