If you already have everything set up as intended and you're not using Cross-Save, then you can safely disregard the information box. If you are using Cross-Save, and have already been playing fine, then you can still safely disregard the information box. Bungie is still trying to help people make sure that they have Cross-Save enabled before logging in on the alternate characters after purchasing Shadowkeep to avoid the issue of that person applying their Season Passes to the wrong save data. You may get the popup multiple times, especially during or after maintenance, when changes are made behind the scenes.