I confirm that in QGIS3.x the "Offset Curve" tool only performs interactive offsets from the current layer.
This is probably an intended behavior in order to avoid duplicating feature across layer, and is unlikely to change in upcoming QGIS versions.
A workaround for your situation:
- From the "Layer" panel select the original layer (the one that contains the features you want to offset)
- With the selection tool, select the polygon(s) you want to offset
- Copy (ctrl-c) or cut (ctrl-x) them
- From the "Layer" panel select the new layer (i.e. the one you want to place the offsets into)
- Switch it to Edit mode (with the pencil icon, or from "Layer"->"Toggle Editing")
- Paste the previously copied/cut features (ctrl-v)
- Use the "Offset curve" tool
You will end up with the offset polygon(s) in the new layer.
Beware that if you "Copy" instead of "Cut" the original feature(s), the original layer will also retain the original features.
This might be OK in your case, but in general I would advise against duplication.