What is today’s equivalent of a world system?

World system in the context of the likes of Saṅkhār’upapatti Sutta, I believe, are humanoid planets.

From what you can infer from Kosala Sutta 1 this looks like a galaxy:

Bhikshus, as far as the sun and the moon revolve, illuminating the quarters with their light, there extends the thousandfold world-system. In that thousandfold world-system there are
a thousand moons,
a thousand suns,
a thousand Sinerus,
the kings of mountains,
a thousand Jambu,dīpas [Jambul Continents],
a thousand Western Goyāna continents [Apara,go,yāna],
a thousand Northern Kuru continents [Uttara,kuru],
a thousand Eastern Videha continents [Pubba,videha],
four thousand oceans,
four thousand maharajahs [emperors],
a thousand heavens of the Cātum,mahārājika [the 4 great guardian kings],
a thousand heavens of Yāma [the Yāma devas],
a thousand heavens of Tusita [contented devas],
a thousand heavens of Nimmāṇa,ratī [the devas who delight in creating],
a thousand heavens of Para,nimmita,vasavatti [the devas who lord over the creation of others],
and a thousand Brahma worlds.
And in that thousand-fold world-system, MahāBrahmā is regarded as the foremost.
Yet even in MahāBrahmā there still is uncertainty, there is change. Seeing this, the instructed noble disciple is revulsed with that.
Being revulsed with that, he becomes dispassionate toward what is the foremost, not to speak of the inferior.

Also see:

• Parallel universes by Piya Tan
• Buddhist Cosmology by Ajahn Sona
• THE BUDDHIST COSMOS: A Comprehensive Survey of the Early Buddhist Worldview; according to Theravāda and Sarvsātivāda sources By Punnadhammo Mahāthero

Based on Piya Tan's Parallel universes, I'm not sure whether there is a modern equivalent term -- because if a type of object is real (e.g. "tiger", "dog", "grass") then I'd expect to find a word for it in several languages -- but if it's mythological then not (because different cultures don't have the same myths).

I doubt it has anything to do with contemporary astrophysics.

I'd avoid calling it a "system" (which a contemporary audience might confuse with e.g. "a solar system").

I imagine it's something like a "horizon" -- but in the second sense of that word:

1 The line at which the earth's surface and the sky appear to meet.

‘the sun rose above the horizon’

2 (often horizons) The limit of a person's knowledge, experience, or interest.

‘she wanted to leave home and broaden her horizons’

I might also term it as a "field of view" -- like someone might say for example that, "microscopic and macroscopic are different fields of view".

Interesting question.

Remember modern day language, the scientific method, Newtonian physics, and higher mathematics only came into existence relatively recently in history around the 17th-18th Century whereas the Pali canons were orally transmitted more than 2,000 years before that.

So understanding exactly what's meant in more modern day terms is tricky but the suttas give us some hints:

“Ānanda, a galaxy extends a thousand times as far as the moon and sun revolve and the shining ones light up the quarters.

In that galaxy there are a thousand moons, a thousand suns, a thousand Sinerus king of mountains, a thousand Indias, a thousand Western Continents, a thousand Northern Continents, a thousand Eastern Continents, four thousand oceans, four thousand Great Kings, a thousand realms of the Gods of the Four Great Kings, a thousand realms of the Gods of the Thirty-Three, of the Gods of Yama, of the Joyful Gods, of the Gods who Love to Create, of the Gods who Control the Creations of Others, and a thousand Brahmā realms.

This is called a thousandfold lesser world system, a ‘galaxy’.

A world system that extends for a thousand galaxies is called a millionfold middling world system, a ‘galactic cluster’.

A world system that extends for a thousand galactic clusters is called a billionfold great world system, a ‘galactic supercluster’.

If he wished, Ānanda, a Realized One could make his voice heard throughout a galactic supercluster, or as far as he wants.” (AN 3.80)

The words "sahassadhā loko" or thousandfold-world is translated as "galaxy" here.

Looks like to me "world-system" or lokadhātu more refers to something like a solar system rather than the entire universe since thousandfold up to billionfold world-systems are mentioned.

It's mentioned that there are thousands of moons and suns in the thousandfold world system more similar to a galaxy.

But remember this was written long long before the concept of a galaxy was ever mainstream so it's still unknown exactly what's meant.

In my opinion certain terms should remain untranslated as it's still unknown exactly what's meant in modern terms.

When debating Menander Nagasena says:

"This world system, O king, is a one-Buddha-supporting world; that is, it can bear the virtue of only a single Tathāgata. If a second Tathāgata were to arise the world could not bear him, it would shake and tremble, it would bend, this way and that, it would disperse, scatter into pieces, dissolve, be utterly destroyed." (Mil 6.1.1)

Indicating that the Earth would explode if two sammasambuddhas appeared at once which indicates that the "world-system" word is more related to the Earth and this solar system rather than the entire universe.

Also indicates that on other world-systems different Buddhas can appear more similar to Mahayana Buddhist concepts.

I don't think it refers to parallel universes as an alternate dimension of the sensous realm because the world systems shake when the Bodhisatta gains final attainment which would be strange if an eventuality caused effect over multiple worlds which are supposedly closed systems with laws of conservation, essentially closed systems of information. To me an eventuality causing a tremble over a multitude of such systems is philosophically like magical thinking as far as i can tell.

Also when the Buddha delivers a discourse such as the Brahmajala Sutta the ten thousandfold world system shakes.

Furthermore given that afaik i've never seen a declaration of a thousandfold multitude of formless realms i think it is the thousandfold world system that undergoes expansion and contraction and the vastness of the universe of form explains the variety of sub worlds which could be planetary systems or galaxies.

Another point is that there is only one Buddha in a world system simultaneously (mn115). Which is significant because if Buddhas can supposedly teleport and if one followed an intepretation of one Buddha per solar system (as some people do) then a Buddha could meet another and this would be contradictory.

I think there are the 31 planes and it is a single expanding & contracting field of form that is inhabited by a multitude of galaxies with suns and habitable planets. I also think there is only one Maha Brahma per expanding system and one Buddha at a time.

Another point worth mentioning is that during a world system contraction, beings are said to be for the most part inhabiting formless realms.