The release of JTS 1.15 officially under LocationTech with a new more compatible license is imminent. Like tomorrow? That soon. I'd like to get a Spatial4j release using it in 1 month. I don't have much time to shoot for more objectives than that with the release but perhaps others do.
I tried JTS 1.15.0-SNAPSHOT (build locally from JTS master branch) with Spatial4j master branch and it worked :-) Mainly I got everything to compile merely by replacing "com.vividsolutions" with "org.locationtech" everywhere. It wasn't quite this easy because I first had to update a Spatial4j test dependency on https://github.com/jeo/jeo to do the same and to update it's dependency on JTS 1.15.0-SNAPSHOT as well. I happened to have that project locally checked out already so I made these changes locally.
Justin, do you think you could do an update of Jeo ASAP after JTS? I may have to submit an Eclipse CQ for it as it's an updated library. I attended a LocationTech PMC hangout call today and it was also suggested that another option is removing the test dependency on Jeo... which I'm not eager to do but yeah it's an option.