Many games have a playfield made up of hexagons. Some games also contain interlocking tiles/sheets that are made up of hexagons (e.g. Clans of Caledonia, Wombat Rescue).
I'm not aware of any that use plastic sheets, other than Heroscape. Nor am I aware of any that use an interlocking mechanism like Heroscape used. But it's interesting to note that the size of Heroscape's hexes is a very common size -- I have not checked this for my answer but I remember them being almost exactly the same size as Memoir 44 terrain tiles?
I'm not a lawyer, and frankly this is definitely the kind of thing I'd suggest a lawyer for, but in terms of prior-art I'd say:
- Cardboard hexes/hex sheets are completely safe
- Plastic hexes/hex sheets are probably safe
- Plastic hexes/hex sheets the same size/shape as Heroscapes, with the same hex size, and the exact same interlocking mechanism is a no-no.
So you'll be able to design and produce your game using similar tiles with no legal issues. And you'd need to consult a lawyer about producing "Heroscape-compatible" tiles.