While @pmanca and I can and have passed this feedback along internally, the HubSpot Community Ideas Forum is the best place to share feature requests. The product teams regularly monitor posts and generally prioritize ideas which have the most engagement from and greatest potential impact on users. Indeed, custom property search is currently available in the HubSpot app because of user feedback on this post.
I found this idea, but my upvote today was the first. If you would like such an API endpoint added, please upvote and comment on the post.
I recommend including as much detail as possible per these best practices. The more the product teams understand a feature's use case, the more accurate they will be when scoping the feature.
Thank you for your understanding. I will close this topic to maximize feedback on this idea.