ArcGIS Online Changes Hatch Color

ArcGIS Online uses the ESRI JS API 3.26 and can use the symbology that's available through the API. Here is a link to the different symbology you can use. You can change the renderer of the hosted feature layer by updating the definition. I haven't tried everything in the sandbox but so far everything works out. This doesn't solve the problem of limited symbology but it gives a bit more freedom.

There is a very simple workaround, one already mentioned in the comments above, which is the picture fill symbol.

All you need is a picture like this:

It's important that it be grey, as you'll see. And that we include the little notches in the corners.

Simply set the symbology to picture fill, then adjust the tint to match the outline color. (If it were black lines in the image, changing the tint would do nothing.) Then publish!

Notice that sometimes AGOL will adjust your picture fill. The hatching I ended up with post-publishing isn't quite what I configured in Pro, but it works. Perhaps you can achieve better results by adjusting the size of the image itself.

You can see the results here.

Finally, if you're working with ArcGIS Pro 2.6, note that you can open, adjust, and save a web map all within the application. This gives you the ability to try and and adjust more complex symbology without having to re-publish the layer every time.