I don't know why I didn't post about Babelfish for Postgresql when it came out a while ago, but the topic came up today during lunch with a friend, so... better late than never?
If you're not familar with Babelfish for Postgresql, it's an extension for Postgresql that gives you wire-protocol and T-SQL query compatibility with Microsoft SQL Server.
In plain English, it lets your Postgresql server pretend that it's a Microsoft SQL Server. What does that mean? It means Postgresql can be a literal drop-in replacement for SQL Server, sans any licensing fees.
Should you just decomission all your SQL Server instances and replace them with Babelfish enabled Postgresql? That depends. Short answer is probably still "no" for the time being.
If you're using enterprise software designed for MSSQL, your vendor (ahem, IBM) will probaby not provide any support if you're not using MSSQL or another approved database server. So don't be using Babelfish for PG as your Cognos content store any soon.
On a related topic, FerretDB is a similar project in that it makes Postgresql a MongoDB compatible server. In other words, FerretDB makes your Postgresql server "web scale" (audio on that link is nsfw).