From what I found tildes was created in 2018 (https://gitlab.com/tildes/tildes/), lemmy was created in 2019, kbin I think started in 2023.
Edit: I just noticed that tildes is under AGPL, and doesn't look like they don't require contributors to surrender their copyright, so it means the code will need to stay open source even if something would change with the company.
somas