The vast majority of Minecraft modders have one big repository divided into multiple branches, rather than multiple repositories with only one branch (as it is in your case).

You could do the same and make a new repository or rename one of the existing ones, and then create in it one branch named e.g. "1.15", which will contain all the resources that are now in the "Gems-and-Crystals-1.15" repository, second branch named e.g. "1.14", which will contain all the resources that are now in the "Gems-And-Crystals-1.14.4" repository, and so on. If you decide in the future to port this mod to Minecraft 1.16 (once Forge is adapted to that version, of course), you can simply create another branch named "1.16". Much simpler, isn't it? ;)

With only one repository it's easier to track changes, report bugs that exist in multiple mod "editions", create pull requests with e.g. new translations, or even bookmark the repository in your web browser.

Even if the 1.15 "edition" of this mod much differs from the other two, IMO division based on branches (instead of repositories) is sufficient.

It is already done, so I'm leaving this as "closed".