Learn English – There is + the (with nouns like “the floor”)

I suppose if you wanted to say something about the "20s decade" of any century, you could say, "I love 20s" or "20s are periods of change" or whatever. But it's such an unlikely thing to say, I'd be surprised if you could find an example.

But to take a more likely analogy, suppose you were talking about the basement of a building. If you were talking about the basement of one particular building, you might say, "I clean the basement." You use "the" because you are talking about one particular basement. But if you wanted to talk about the basement of any building, you could say, "I clean basements." Now there's no article because we are talking about basements in general and not one particular basement.

As "the 20s" would normally be understood to mean "the 1920s", we are talking about one particular period of time, so you use the singular and the definite article. (I suppose if someone in 1840 talked about "the 20s", he presumably meant "the 1820s", and if 30 years from now people talk about "the 20s", they'll probably mean "the 2020s", unless the context makes clear that you're talking about some other century.)

Note that regardless of whether you are talking about a specific thing or a general thing, if you use the singular, you need an article, a possessive, the number "one", or one of a few other special pronouns. Like if we are searching for your lost dog, I might say, "I see the dog" or "I see your dog." If we are wandering about and see an unknown dog, I would say, "I see a dog." It would never be correct to say, "I see dog." (Well, you could say "I see Dog" if you are talking about Dog the Bounty Hunter, a person named "Dog". But then it's a proper noun.)

As Mick notes in his comment, you could use "20s" without an article if you were using it as an adjective rather than a noun, and the noun in question does not call for an article. Which would usually mean that it is either plural or uncountable. "I like 20s music" (uncountable) or "There were several 20s cars in the antique car show" (plural).

As described here, it is idiomatic to use the zero article in some possessive phrases which refer to a thing as representative of sub-class within a class:

• this type of guitar
• that make of car
• what sort of plectrum

It is also idiomatic to use the definite article with the singular or plural noun when that noun refers to a thing as representative of an entire class of things.^†

Examples abound:

• The capo d'astro is affixed to the neck of the guitar.
• The computer has changed our world.
• The Dodo is an extinct pigeon-like bird.
• An example of the order Rosales is the rose.
• This pathogen infects the rose tree through the cut surface of the graft.¹
• A shoe is a covering for the foot.
• The unicorn is a mythical beast.
• Multiple plectra plectrums are worn on the fingers.
• "The rose looks fair, but fairer we it deem..." ^‡

^†_{Note that this is true of all nouns, not just those which refer to parts of the body.}
^‡ _{William Shakespeare, Sonnet LIV}
1_{GEMES-ROSES - Disease and pest control}