Thanks for your bug report. This is due to the WinGet repository not having been updated to download mIRC 7.79 yet. I have never used or updated this repository myself.
The reason WinGet is failing is that the mIRC website only hosts the latest version of mIRC and redirects old download links to the download page so that users download the latest version.
For the time being, I have re-enabled the most recent version download links so that WinGet works.