Hello safrick!
When the system restoration is complete, all stations’ legacy data will be accessible until the first day that station comes online. So, for example, if a station came online in 2018 and was never disconnected, you would have data from the present day back six years now.
As of today, legacy data is accessible back to January 1st, 2022 (as you can read in the last messages of the updates topic: Live Data Issues). This data is accessible from legacy data platforms such as DataView, FDSNWS, and others.
To remain always updated on our progress, the link above is the place to go for all the latest.