I guess this had to happen. The writing has been on the wall from the start that this was not going to continue otherwise we would have seen it further along the timeline.
I like the reason given, that Stamets informs us that the use has been stopped until a non human interface can be developed, and the inference is that that will never happen.
I think this is a good idea as the continued use and investigation would allow for massive problems for temporal issues.
Jumping around in time can really mess with your canon as can going anywhere instantaneously, sometimes you need the time imperative to drive a story line and the spore drive would kill it. Oh this planet has some epidemic and Disco needs to get a vaccine there as soon as possible. Well ok engage the spore drive and put us there a week before the epidemic started.
It was an interesting concept but I think it would have killed the show if it had continued.
What do you all think?