Yes, the floor is never a good choice. Restaurant floors see so much foot traffic from shoes that have been outdoors or who knows where, so it’s best to keep our bags well away from that.
Placing it on the lap can feel awkward and impractical, especially with a larger bag, and it interferes with properly using your napkin (which belongs there for good reason).
The table itself isn’t ideal either as it’s where our food is served, and purses aren’t cleaned as rigorously as we might hope, so hygiene is a real concern. The table is a place for food and drinks, not bags.
The most practical solutions are exactly this: a small stool or side table that thoughtful restaurants provide for our handbags, or a discreet pull-out ledge built into the chair. When those aren’t available, I usually tuck a smaller bag behind me on the chair.