For me, the solution turned out to be completely different than the above answers. The split icons did not show up, and I desperately searched for a solution, but not with AndroidStudio, but with IntelliJ.

I was using IntelliJ IDEA 2024.3.2 (Community Edition), and I have installed the following IntelliJ plugins:

• Plugins: Android
• Plugins: Jetpack Compose

What I was missing was the following IntelliJ plugin

• Android Design Tools

After I installed this plugin, the split icons Split | Design were working again.