I'll write up a guide on how to build the Textual app version 7.x by pulling the source from GitHub. It's a fork of Limechat. In regards, to your searching for Textual version 6.x. I didn't realize it was still available but you can acquire it using Homebrew Cask. After installing Homebrew and then the tapping for Cask, then you can initiate a search for "textual". The version 6.x is actually the "textual-beta" cask.
BTW. If you were already wanting to build and having an issue for Code Signing, then give some more insight. It's fairly straightforward, as long as, you don't have any restrictions on your account, IP address, etc. which would keep you from being assigned one. You request a certificate via Keychain Access.
Instructions to build Textual from a clean project directory. If you've made attempts to build and they have failed, then you most likely have an incorrect configuration file for Xcode or your project. It's best to just start from the beginning. Research how to reset Xcode and remove any derived data from previous Textual builds with your project.
Prerequisites: Xcode 10.0 or newer (App Store), Code Signing Certificate (Keychain Access)
0. Open terminal and create a build directory to clone the source from GitHub.
1. Navigate to the build directory in terminal.
2. Clone Textual from GitHub using terminal:
3. Navigate to the newly created Textual sub-directory of your build directory using terminal.
4. Pull the software modules which Textual is dependent upon:
git submodule update --init --recursive
5. Navigate to the sub-directory "Build" which should be located at "(Created Build Directory in Step 0)/Textual/Configurations/Build/".
6. Open the file "Code Signing Identity.xcconfig" with your choice of text editor. Example: pico. (Xcode is not a text editor)
7. Edit the "CODE_SIGN_IDENTITY = Mac Developer: (Enter your code signing identity from your requested certificate stored in Keychain Access)"
8. Edit the "DEVELOPMENT_TEAM = (Leave or delete characters until it is empty)"
9. Edit the "PROVISIONING_PROFILE_SPECIFIER = (Leave or delete characters until it is empty)"
10. Save the changes to this file and close the text editor.
11. Navigate to the sub-directory "Standard Release".
12. Open the file "Enabled Features.xcconfig" with the previously used text editor.
13. Edit the the following string to match "TEXTUAL_BUILT_WITH_LICENSE_MANAGER=0".
14. Save the changes to this file and close the text editor.
15. Open Xcode and open the file "Textual.xcworkspace".
16. Xcode will index and process files for building your project. After this is completed, select "Textual (Standard Release)" from the menu for your workspace.
17. The menu for Project target in the workspace window can be "Textual App".
18. Select 'Product > Build' from the Xcode menu.
19. You should receive confirmation that the build was successful.
20. Navigate to "(Created Build Directory in Step 0)/Textual/Build Results/" and copy the Textual.app to your Applications folder.