Change Table Field Editable to "yes" on a Hosted Feature Layer

I believe this to be one of the many functions that does not exist on AGOL, to be able to say if a field is editable or not.

You have NO because when you used ArcMap/Pro to upload the layer the field in the map document had it's Read Only property ticked on, so not editable. To remove this property in AGOL you need to change the property of that field in your map document and then upload the layer overwriting the existing one.

Make sure you have synced any data that has been created in that layer whilst it has been on AGOL, otherwise when you overwrite you will loose the data.

If you are the admin for the Feature Service, it is possible to change this property directly in ArcGIS Online by editing the schema of the layer. To do so, you may find the instructions in the following guide helpful:

https://community.esri.com/ccqpr47374/attachments/ccqpr47374/water-utilities-questions/376/2/How-to-Update-Hosted-Feature-Service-Schemas1-2.pdf

In the JSON, you will want to scroll to the field and then change the "editable": false, line to "editable": true,.

After you change this property, you will then need to go to the configure pop-up pane for the layer, click on "configure attributes", and check the box in the edit column. Press OK, save the layer (as opposed to the map), and you should be good to go!