[atls]

It's not a complete replacement, but if you're in a Python ecosystem, you might find Moto to be of interest.

https://github.com/getmoto/moto

[henriks]

at least some parts of localstack seemed to be built on moto, based on a brief look at some point

[switchbak]

They were, but they moved off them - probably with a move away from OSS as a long term plan.

I've used Localstack extensively for ~7 years, and I will rejoice when I can finally be free of it. I've found it to be low quality software, and full of bugs.