I produce videos that are mainly audio with some slides as visuals.
The slides change about once every 30 seconds, so even at a frame rate of 1fps my video would play smoothly.
When I try and export my video from Premiere Pro the lowest frame rate h.265 encoding allows is 10fps. Is that limit put in place by the Codec?
To answer the title question exactly, no, the theoretical floor for H265 framerate is probably less than 1fps. But you don't want to fiddle with framerate anyway. It won't help you produce a quality video within your spec requirements (unless some unusually low framerate is in your specs).
When making videos from slides presentations is tempting to believe that you will gain meaningful file size reductions by lowering the framerate, but this notion is completely false. It is also false that you will gain meaningful quality increases with no size increase.
H265 is very good at interframe compression. If you take a static image and make a 30fps H265 video from it, the resultant file will be pragmatically identical to 1fps, or even 60fps versions.
Continuing on pragmatism, most end user players I've looked at for this issue struggle to play framerates under 15fps. In my experience, your videos won't play smoothly at 1fps. Any quality gains or size reductions are meaningless if end users cannot play the file.
You need to render your file with a variable bitrate. I'm not familiar with Primere Pro; and complete control over your renders, as can be attained from ffmpeg, may not be available, but I'm sure you have a VBR option. Try it out, and leave the framerate at the standard 30fps. Optionally, render for your permanent archive, then process with ffmpeg for end user distributions.
