This doesn’t sound like an issue for those who use Fusion frequently, however you may want to find ways to get local files, just to be safe.
https://github.com/aconz2/Fusion360Exporter lets you bulk export all of your projects to local storage.
It exports as F3D (probably readable by future versions of Fusion) and STEP (standard but with less fidelity) by default.
Is STEP a CAD format or 3d file format? I thought the latter. Like I don’t get constraints or sketches, only 3d models