This is a dead simple data pack that removes Tinkers' Construct's Slime Islands from the overworld (Nether and End are left intact).

Install Recommendation:

Use a mod like Open Loader or Paxi and drop this pack in the data folder (Twitch\Minecraft\Instances\<ModpackName>\config\paxi\datapacks), so that it automatically is installed in each new world.

Optional Info: I couldn't figure out why the TConstruct config mentioned geodes in the nether but not the islands so I poked around github and found the info below. Unfortunately I had no idea how to make a data pack at all, much less to edit a mod .jar or what lines to edit or delete so I bumbled through learning all that--and I'm sharing it here in case it saves someone else a couple hours of their life.

If you want to delve into Data-Packs-As-Configs more then I recommend heading over to the Repurposed Structures page and scroll down to 'How Can I Configure This Mod'. They really made my newbie life easier.

Via Dev's GitHub: Slime islands missing from config #4868

No they are not. They have been moved to vanilla's config. It is an absolute pain now to add a config to disable a structure in forge's config because that means adding a config on top of vanillas config. Look in the mod jar, you will see some JSON files for the slime island placement. A datapack can change those.

Honestly, there are probably 4 or 5 ways to disable it, including emptying the biome tag for valid biomes for the island, emptying the list of placable strucures, etc. This is the direction the vanilla game is moving and its a direction I agree with, in the future nothing in tinkers will be in the config file as everything configurable datapack controlled.

Tinker's Construct Slime Islands: Disabled