Is there an Ethereum API where I can drop an arbitrary number of Ethereum addresses and then see a reverse chronological feed of all their on-chain activity?

I've done that with zapper.fi, I created a group of addresses and it coalated them into a single feed

May not exactly fit your use case but this app is pretty neat to visually see on chain interactions of DAOs and their participants.

https://pod.xyz/

u have to specify the on-chain events you’re looking for to constrain the query. some useful APIs to check out - spec.dev, goldsky, https://coherent.xyz

You can do this on Airstack, depending how you define "all" on-chain; 

and whatever is not there rn we could add quicklyl

in your use case, which activities would be included?

- transfers

- sales

- mints

- DAO votes?

- contract interactions?

Let’s chat and figure out what you need! ... longcast.jam.so/jjOmaq

Etherscan has this but for a single address https://docs.etherscan.io/api-endpoints/accounts#get-a-list-of-normal-transactions-by-address

Checkout Moralis API. Lots of good stuff and easy to use