You can store as much data as you want for as long as you want in Graylog Open. However, the more data you store in Opensearch the more resources it requires (not just disk space).
In practice at 30 or even 90 days that is totally worth it, but longer than that the chance you will look at those logs goes down, and therefore the cost to value goes up.
What the archive feature in the commercial versions of Graylog gives you is the ability to take that data and move it “offline” in a compressed format, but then later you can easily restore that data back into Graylog if you ever need to work with it.