The possibility to activate legacy extensions (such as Brush box) is not visible in PS 2025 anymore, compared to 2024. It is not greyed out - it is just completely removed.
Check your plugins folder to see if they are there. If not either reinstall them for 2025 or manually move them to the same folder location in the 2025 app folder.
So it turns out I had to start Photoshop in Rosetta mode to get the extensions to even show up. However, they do not work since a JavaScript element is missing. Is there any solution to that?
Legacy extensions do not work on ARM systems in native mode, you have to use Rosetta mode.
There is currently a bug where switching modes disables a variety of features in Photoshop. You can fix it for now by simply resetting preferences. We've got a full fix coming in a future update.