Some regions and states have incentive programs to fund work that minimally delays stormwater from hitting sewage systems. Features like rain gardens mentioned above often qualify.
For example in Portland OR there is Clean Rivers Reward Program.
In my town, a suburb outside of Chicago, a friend had a sizable chunk of landscaping cost rebated by incorporating stormwater runoff retention features.
It might be worth investigating if any such programs apply by you.
Could fill it with gravel a little at a time. That way you don’t have a mess of stinky water going anywhere. If you just toss a few shovelfuls of gravel into it every day for however long it takes to fill up, the gravel will slowly force the water out and the water will go away on its own without making too much of a mess, since only a few shovelfuls of it get displaced each time.
Presumably you’re going to put some sort of cover over it in the meantime; sturdy enough to keep children, dogs, etc. from falling in.
Yeah, I was thinking that would probably be illegal here now (at least unless the neighbor owns a great deal of adjacent land, part of the same lot, that isn’t in impervious coverage); but that’s relatively recent legislation.
No. Absolutely not.
People will throw things in there that shouldn’t get into the groundwater.