Please know that I'm not affiliated with the OSM Foundation or data working group - I'm a member of the public with an interest in open source maps. There is no-one to 'escalate' the issue to unless an OSM-contributor suspects vandalism to the map database has occured. I would recommend you familiarise yourself with the concept here:
https://wiki.openstreetmap.org/

For clarity, the new pin you've just made at the correct location (https://www.openstreetmap.org/note/5097398), but let's continue the discussion here for convenience.

The route of the Frome into the floating harbour before the construction of Mylne's Culvert is represented by the following way:
https://www.openstreetmap.org/way/1449669104

This was, until a month ago, included in the River Frome relation, based on OS maps from before Mylne's project and which are no longer to copyright, but are, as you point out, no longer up to date.

The Frome relation now includes Mylne's Culvert as it is, as you say, the course which the water follows under normal conditions:
https://www.openstreetmap.org/relation/2863283

I mapped this route from design drawings of Mylne's Culvert after receiving permission from Bristol Archives to do so. These drawings show the route approximately (probably within 10m).

If you have access to information which could improve the accuracy and is compatible with OSM's terms of use (https://osmfoundation.org/wiki/Licence/Contributor_Terms) - fantastic! I would suggest familiarising yourself with how to edit the map and correct it directly yourself, acknowledging the source of the changes and including in the changeset comments a confirmation that the source meets the OSM's Contributors Terms