The Plugin keyframes are structured. The individual data values are named parameters to the keyframe function. For example, the hue plugin has keyframe parameters of hue, saturation, and value. Each plugin has its own parameters, and what they do depends on the plugin. Most of the time, it is pretty obvious what the value controls, like the audio gain plugin with the level parameter. Some of the plugins have a wide variety of controls, like the titler which can setup a wide number of controls, like formats, fonts, styles, placement, and so on.
Plugins also may use interpolation to smooth the data between the keyframe paramaters on the timeline. Just exactly how this is done varies with the plugin and is not always easy to predict, but this is usually due to the nature of the keyframe parameter data. For example, it does not make sense to interpolate which font is in use.
Plugin keyframes are handled inside its own plugin. CINELERRA-GG knows nothing about internal functionalities inside plugins, either external (such as ffmpeg) or native plugins. And has no chance to do any interpolation. Only the plugin can interpolate its own keyframes. And if you succeed to add plugin keyframe interpolation in some particular plugin, all the other (even similar in functionality) plugins will know nothing about it. So, each one out of dozen of plugin keyframes has to be implemented individually.
The CINELERRA-GG Community, 2021