given we already migrated another place to store content (T183489: MCR schema migration stage 1: Fix Legacy Archive Rows)

That task was about finally migrating a relatively small amount of old data from before MediaWiki 1.5 (circa 2005) to the new format introduced back then. It doesn't seem particularly relevant to this task.

and the text table itself is going to be emptied soon in Wikimedia wikis (T183490: MCR schema migration stage 4: Migrate External Store URLs (wmf production)).

The current status is that content.content_address refers to a row in text, which in turn references ExternalStore for the actual storage. That task is about having content.content_address reference ExternalStore directly. It wouldn't touch the actual data in ExternalStore.

This task would require actually updating the data stored in ExternalStore, so that isn't relevant here either.

More relevant tasks would be T181555: Remove use of PHP serialization in revision storage or T106386: Compress data at external storage, which do require updating data in ExternalStore.

It seems Wikimedia is still using it (cur.ibd in T54921: Database tables to be dropped on Wikimedia wikis and other WMF databases (tracking)), they should be migrated.

Indeed. Actually doing so is the part that needs someone to do a decent amount of work, as shown by the amount of planning that went into T181555 (currently stalled on people (e.g. me) having time to get back to it).