In the US there are also things generally called “private roads”. Which as the name suggests, is a roadway built on private land. Which may well be passable to everyone. These often lead into a small group of houses who collectively own that right of way. They’re often restrictively signed to exclude anyone who doesn’t have need to be there. But that’s not really a practically enforceable exclusion.
The driving areas within a large commercial parking lot are legally similar; public access, but not owned by the local government, and technically subject to the owner’s right to exclude people lacking legit need to use them.
In all those cases, the city is frequently not on the hook for pavement maintenance, nor for litter removal. That’s entirely on the landowners.
I would not be surprised to learn that your alleyway is similar. It is within the local council’s geographic purview. But at the same time it’s private land belonging to somebody and they have no more duty to remove rubbish from it than they have duty to remove rubbish dumped on your doorstep, nor to trim your hedges.
I don’t think this is equivalent to the American phenomenon the OP asked about. AIUI, there is no “unincorporated” land in England (I’m talking about England here - local government is a devolved matter so may be different in Scotland, Wales or Northern Ireland). To my knowledge, every point in English territory is assigned to some local authority. The style of this local authority may vary - some are called “cities”, some “districts”, some “boroughs”, and some are “unitary authorities” where local and county functions are merged into one, but they’re all “local authorities” in legal terminology. Rather, “unadopted” status refers to a particular road that is not maintained at public expense, but the land on which the road lies still belongs to some local authority (see link).
You don’t really see cities “unincorporate” bits of land much — more like boundary reshuffles forced from above. Most local govs won’t agree easily coz it hits their tax base pretty hard.
In Canada, the municipality of Headingley seceded from Winnipeg in 1993. The area had been amalgamated (along with a few other cities & suburbs) into the City of Winnipeg in 1971. However, the city struggled to provide the same levels of services to a primarily rural area as they did to the rest of Winnipeg, and Headingley residents chafed under the additional tax burden. In a 1991 referendum of Headingley residents, 87% of voters were in favor of separating from Winnipeg.
The amalgamation and the secession were both done through acts of the provincial government; neither the city of Winnipeg nor the residents of Headingley had the power to “unincorporate” part of Winnipeg on their own.
The Fraser Institute, a Canadian libertarian think-tank, wrote a white paper about the process of “de-amalgamation” in Canada if you want to delve into the gory details.