Will there be any side effects if all dimension styles are purged from the file or the Dimension Styles > Dimension Lines > Level setting is switched off and the respective level (G-ANNO-DIMS) is deleted from the file?
Meaning the definition is not being used by dimensions on that level, then yes it would be safe to purge them out. Purging them out will then allow the Level to be deleted if the usage for that level shows that it is being used by a dimension style. Check the usage for that level and confirm that it is indeed a dimension style definition that is being used by the level.
Go to Settings > Level > Manager > Right click on the level, then go to Properties > Usage. This will show how the level is being used, whether it is a text style, dimension style, shared cell etc. Once you know what is in use on the level you can then make the determination of purging out those definitions.
If those dimension style definitions are still in use and you delete them, you will get the following message " Style <(><<)>Name of Style> is in use. Deleting style will drop style association from dimensions. Do you want to proceed?" If you choose to proceed your dimensions that are in the file will no longer have association to that style.If Styles > Dimension Lines > Level setting is switched off, this will mean that instead of the dimensions being placed on this level, they will go on the active level rather then that specified level through the dimension style. So if this level is deleted you would want this turned off so that the dimension is not trying to be placed on a level that does not exist. Deactivating this option is not the cause of the level not being allowed to be deleted. It is because there are elements that still exist on that level or definitions of elements that have been deleted still exist.