Soul speed is not incompatible with depth strider and/or frost walker

this is a feature request please use the minecraft feedback website for future feature requests

https://feedback.minecraft.net/hc/en-us

Personally I would make soul speed incompatible with depth strider but not with frost walker, but this suggestion was not implied at all in this report. It would be nice to get some confirmation on whether this is intended, which is why this report remains open.

This is not a bug. Frost Walker and Depth Strider have conflicting effects, which is why they are mutually exclusive. Soul Speed doesn't conflict with either of them, hence why it isn't mutually exclusive.

this may be sort of off-topic, but in that case, why are mending and infinity incompatible?

There's already discussion on mending and infinity being incompatible on the feedback site. If you want to give feedback or suggestions for Minecraft, please make a post on the feedback website, or if a post on the topic already exists, make a comment on said post: https://feedback.minecraft.net/hc/en-us