[windsurfer]

Even if you remove a watermark, the companies still have a record of which images they have generated and for whom. Even if you remove the obvious watermarks, all major image generators are using steganography to embed hidden information that you can't be sure were removed. This is a type of one-sided arms race where one player gets to be invisible if they want to.

[swingboy]

Do they still work if you apply something like a filter or additional layers on top? Or add a subtle blur, etc.

[windsurfer]

Yes usually, since an important aspect of steganography is error correction. For example we know that SynthID is robust enough to survive resizing and small blurs.