Is "nearbys" the right word to express "places/ objects that are nearby"? Or does a better word exist?

To answer the specific questions, OED does have nearby as a noun, and its plural is nearbys, but it doesn't mean "a nearby place". It means a futures contract which is nearing maturity.

Stock Market.

A nearby futures contract (cf. nearby adj. 2). Usually in plural.

1980 Times 30 Dec. 16/6 Wall Street... Cotton futures eased from strong opening levels to close with gains of 1.17 to 0.75 cents in nearbys.

The adjectival reference says

2. Stock Market. Of an option or futures contract: nearing maturity; spec. that expires in the next month of active trading in that particular type of futures. Cf. nearby n. [quoted above]

That said, you can certainly refer to nearby places as nearbys within your program; the context should make the meaning clear. If this is going to be a user-facing term, then it might be best to come up with another word which I'm sure other answers will be able to supply. Nearby as a noun is a jargon term not in a general use.

If we are talking about objects, I would concur with Stuart and suggest neighbors (UK: neighbours) as a more widely accepted term to use.

If you want something a bit broader, then perhaps surroundings will suit you better:

Surroundings

: the circumstances, conditions, or objects by which one is surrounded:

But new research into the health effects of our surroundings is spurring the development of facilities that feel more residential.

I don't know much about your programming context but I can imagine something along the lines of:

Be careful when editing parameters for these objects as they can have unexpected effects on its surroundings

At the end of the day, this is still relies on context, but it is at least a more common English term.

This came up in a programming context. A group of objects, that are nearby another one, was simply called: “nearbys”. These could be actual places/ locations or really random objects. This raised questions: 1. Is this nominalization actually valid or used in everyday language use?

It is common to use nominalised adjectives:

“The sick, the lame, and the lazy came begging for money.”

In their adjectival state, they represent an ellipsis of their plural noun (above it would be “people”) but are usually uncountable as they describe the defining characteristic of an homogenous generality. (The sick = all who are sick)

*“The sicks, the lames, and the lazies came begging for money.”

However, words that commonly function as adjectives can function as common nouns and are countable:

“See those red balls and the blue ones? Give me the red.”

“See those red balls and the blue ones? Give me a red.”

“See those red balls and the blue ones? Give me the reds.”

In your example, nearby is countable, and thus a common noun.

Also would it need to change the y in the plural form?

No. the spelling "nearbys" seems most useful to prevent the pronunciation of nearbies as "nearbees".

The word, as coined and defined, is good: it's meaning is clear and apt. I have no problem with it.

To complete Stuart F's answer to go with neighbors, the noun "neighborhood" can be used. Although this is the whole set of places that is near the point being talked about, so the word doesn't evoke the individual places in the set.

Proximate:

adjective

next; nearest; immediately before or after in order, place, occurrence, etc. close; very near. approximate; fairly accurate. forthcoming; imminent.

https://www.dictionary.com/browse/proximate?s=t