The apps from Andreas Hegenberg are no longer available here.
Legacy versions of his apps (released before February 2018) can be used for free using this license:
https://boastr.net/free_legacy_license.bttlicense ( or click here to activate immediately)
The last BTT version from Andreas Hegenberg can be downloaded here: v2.342, if that one crashes on your system, try v2.428 instead. You might need to manually copy the license file from above to this location if you want to use it: ~/Library/Application Support/BetterTouchTool/free_legacy_license.bttlicense (because without the license the version has expired by now).
However it is not recommended to use these old versions as they can contain security and compatibilitiy issues and won't receive any support anymore. They might also crash on current macOS versions because many things have changed.
A snapshot of the source code (from early 2018) for all of Andreas Hegenberg's apps can be accessed via https://boastr.net/legacy_source_code.zip (MIT license, unpack password: BTT_BST_LEGACY). The SecondBar code can be found here: https://boastr.net/SecondBar_Source.zip
Andreas has since founded a new company (folivora.AI GmbH) which aims to develop more modern versions based on that open source code (and more).
Imprint: Andreas Hegenberg - Auerbergstraße 6, 81547 München. Tel.: +49 170 4817600 - E-Mail: firstname.lastname@example.org