Plenty entered English as a noun from Old French plentee or earlier plentet ‘abundance, profusion’ in the mid-13th c., though the partitive construction appears to be somewhat more recent:
(a1398) * Trev. Barth.(Add 27944) 24b/b: Blak eiȝen moueþ lasse for multitude & plente [L multitudinem] of humours
a1450 St.Editha (Fst B.3) 181: Plentythe of vitel þere was y-nowe.
a1450 St.Editha (Fst B.3) 519: Plenteythe of fysshe þey hadden y-nowe.
Wheat and rye have risen to a mark and 16s. a quarter. There is plenty of old corn and new barley, oats, pease, and beans. — Lord Chancellor Audeley to Cromwell, 13 Sept. 1535 (Letters and Papers, Foreign and Domestic, of the Reign of Henry VIII, vol. ix, London, 1886).
If one admits plenty into the limited set of English quantifiers, such as some, many/much, enough, all or few, then a pronomial use should be uncontroversial:
This competition has forced many smaller companies to go out of business, although plenty are still around to compete with the big names of Pizza Hut, Domino’s, Little Caesar’s, and Papa John’s. — Leonard M. Jessup, Joseph S. Valacich, Information Systems Foundations, 1999.
Tucked away in corners of central Maine, plenty are still crazy about the most popular sport on the planet, Travis Barrett writes. — “The World Cup in the digital age,” CentralMaine.com, 23 June 2018.
My host in Paris once sent me across the city for what she promised was the best eclair, even though there were plenty available right around the corner.
While there are a number of listings run as investment properties, complete with a property manager and call center if you call for help, plenty are still the original Airbnb idea — a room in someone's home, or their house that they vacate for you. — NBC News, 11 July 2018.
If you want the hottest ticket in town [for Hamilton], plenty are still available through Ticketmaster. But be prepared to pay as high as $1500 apiece. — Boston Globe, 9 Oct. 2018.
If you replace plenty with some or many in these examples, you might change the relative quantity of pizza chains, eclaires, or theater tickets, but you won’t be changing the grammar: plenty functions identically to quantifiers used as pronouns.
As far as I’ve been able to determine, however, this use of plenty without a partitive of-phrase, as frequent as it is today, doesn’t seem to predate the 19th c.:
The merchants might as well be employed in the importation of foreign wheat, when plenty is to be procured in the home market, — The Sydney Gazette and New South Wales Advertiser, 27 June 1827.
“Mother wants your sifter, and she guesses you can let her have some sugar and tea, ’cause you’ve got plenty.” — Newburgh Telegraph (NY), 31 Oct. 1839.
I have only about one bushel of the Carolina, but plenty are to be had of Baker & Hamilton, Sacramento city. — J. S. Curtis, “California Pea-nuts,” California Farmer and Journal of Useful Sciences, 1 Feb. 1855.
It is highly probable they will fall in with Indians, as plenty are about. — San Antonio Texan, 12 Feb. 1857.
Even so, plenty as noun is far more frequent in 19th c. sources, appearing in contexts where contemporary speakers would likely say an abundance, a sufficient number/amount of or simply plenty of. One author draws a marked distinction:
Plenty is, however, more frequent in the literal sense for that which fills the body ; abundance, for that which fills the mind, or the desire of the mind: a plenty of provisions is even more common than an abundance; a plenty of food; a plenty of corn, wine, and oil : but an abundance of words; an abundance of riches; an abundance of wit and humour. — George Crabb, English synonymes explained in alphabetical order, 1818 (repr.1978), 736.
This author apparently didn’t get the memo:
A plenty of people die, or survive a long or acute illness but half alive, for want of efficient nursing. — Anna Elizabeth Dickinson, A Ragged Register: (of People, Places and Opinions), 1879, 120.
Pronoun, Yes or No?
While Merriam-Webster only lists plenty as a noun, adjective, or adverb, MacMillan follows Oxford in giving it a pronomial use. I assume this decision rests with whether the editors wished to include the word as a quantifier in standard English or not. CGEL’s always singular “number transparent quantificational noun” never using a determiner sounds so overqualified as to be a subset of one. A few snips of Occam’s razor to this definition would suffice to parse plenty as a noun like abundance or as a pronoun like some or many.