I now finally got my hands dirty with Unity 6.1 & com.unity.web.stripping-tool. You can see a more detailed discussion over here: Unity Web Stripping Tool Feedback - #4 by Johannski, but the high level info is this:
| Base Optimized Build | Stripped Build | |
|---|---|---|
| BiRP | 6000.1.11f1-minsize-webgl2 (2.93 MB) | 6000.1.11f1-minsize-stripping-webgl2-v3 (2.85 MB) |
| URP | 6000.1.11f1-urp-minsize-nostripping-webgl2 (6.11 MB) | 6000.1.11f1-urp-minsize-webgl2 (5.63 MB) |
So it is able to shave off some build size, but since the engine grew as well, the reduced sizes are still bigger than builds with Unity 2023.1 or older.