Short answer: yes it dissuades me from considering you, to some non-trivial extent.
I'm answering this as a hiring manager, but with a big caveat: I firmly believe in hiring the right person for the job, regardless of where they live, and not requiring relocation if the employee has any hesitation about doing so. But, depending on the company I've been with, that has not always been possible to offer, so I do also have experience with this more typical scenario.
When I have selected a short list of candidates to interview, I'm already investing time and mental energy into their candidacy. If, in the course of an interview, I learn of any hesitation on part of the candidate for anything related to the position -- duties, responsibilities, location, whatever -- there is a negative mark against that candidate. The negative mark isn't because they did anything wrong, but because it makes their candidacy less than a perfect fit. Maybe it drops from "this person would be great" to "this person would be great but here's this little nagging thing that we might have to work through -- maybe shift around some duties, make some accommodations, etc".
If there are 10 applicants, 2 are perfect, and 8 are great and have some sort of nagging thing, I am going to spend the remaining of my efforts on those first 2, first, unless there's some ultraspectacular quality to one of the people in the group of 8.
Those nagging things that cause negative marks do vary; hesitancy at performing task X would be weighed lower than hesitancy at relocation. Hesitancy at relocation -- even a little bit, let alone a 30-40% chance that you're going to ultimately have wasted everyone's time no matter how nice you probably are -- would be, to me, the biggest of the red flags in the process.
Not to pick on you by using you as an example, but I will since it's your question...if I were interviewing you, and my company didn't hire remote workers, and you expressed that you were 30-40% sure you weren't going to relocate, I would absolutely not continue spending time with your application unless you were so spectacular that anyone and everyone would be a fool not to drop everything and try to pursue you. There are precious few unique talents like this (see Free Electron). I would pursue (and have) a known Free Electron for the good of my company.
So where does that leave a prospective candidate who also wants to be honest (and honest is always a good thing, and who probably isn't a Free Electron (again, not casting judgement on you, just playing the odds)? I would suggest that until you are 90%+ sure that you would relocate to New Location, don't apply to the position. Especially don't apply to the position if you're just using it for practice -- that's wasting your time, my time, and burns any potential bridge that could've been built for future opportunities.