Still using Tiki 15.
Moving soon into 18 and then the next LTS (I don't get why there is a 2024 LTS version).
There is no 2024 LTS. This is how it works: Lifecycle
You can go directly from 15 LTS to 24 LTS. Install a 24x and point it to a copy of your 15x database. And during the installation, the upgrade script will handle all the database schema changes. In fact, you can upgrade like this from really old versions like Tiki2. Ref: https://gitlab.com/tikiwiki/tiki/-/tree/24.x/installer/schema