There was some sort of v4 -> v5 open-source -> commercial switch.
Commercial version: http://www.poderosa-terminal.com/
OSS version homepage: http://poderosa.sourceforge.net/
Source (last change in 2019): https://github.com/poderosaproject/poderosa
Releases (last release also 2019): https://sourceforge.net/projects/poderosa/files/releases/4.4... (also on GitHub too, switch to the "tags" tab)
Seems to be .NET based, and relatively simple in terms of available functionality and featureset.