Is this rotten?

[Bbbbbbbb2017]

Had a bathroom leak and the bath panel got ripped off to leave this...

Is this rotten?

Looks water damaged and at least mouldy. Does the wood feel soft?

The floor does. I backed off when slugs appeared. I live in a first floor flat so god knows where they came from

Push a key into it in a place a dent won't be seen, if it goes in very easily, the wood will need to be replaced. if not, it should dry out okay in time.

it looks like a chipboard floor, which turns to mush when wetted. It is complete rubbish and only fitted by builders because it is cheap. Look at the feet of the bath. Are they standing on undamaged board? and is it undamaged far enough to be supported by the joists on each side? (look for the nail heads)

If not, one or more feet might fall through the floor. You should replace it with 18mm WBP ply, and preferably coat all the edges with varnish or paint to prevent water absorption. It will be difficult to do this without removing the bath and damaging the tiles, so consider if you can afford to renovate the bathroom. It might be possible to loosen the bolts and raise the feet, one at a time, and slip a ply or metal bearer under them, as an expedient.

It's possible you can claim on your household insurance. Ask them.

If the leak has been fixed, the slugs will eventually die, but you can speed it up by scattering slug pellets thinly.

Leave the panel off so it can dry out.