Avast video downloader is also missing on my PCs. Last used a few months ago, went to use it last week and no longer listed in my extensions. I performed a clean uninstall/reinstall, which did not bring it back. I have developer mode enabled and it's just not there.
Only thing of note in my installation, is I'm not using avast AV, so it's a standalone app. I deselected the option to make it my default browser, deselected launching automatically on Windows login, & deselected pinning it to my taskbar. Otherwise I've not changed anything in the settings. I've attached images showing what I mean and my version. Win 10 Home 21H1 19043.1023. Using with Windows Defender. The avast locale I downloaded from is en-NZ, default language on installation English-UK.
I see this has been reported to avast so I'm just posting to add a little weight/numbers reporting, and a little more info about installation info in case it's relevant. Also to make it clear it's not the same issue as the other post about it just not showing on youtube.