The Boost for Reddit app still worked for a while after the big shutdown with workarounds, like creating a mod account or using revanced.
Not anymore, as of today, the Boost for Reddit app is completely 100% dead now.
Goodbye reddit forever, hello lemmy
Edit: Working again
Reddit did some change in the API that breaks many apps. As far as I know they are aware of the issue and will fix it. More info here: https://www.reddit.com/r/redditdev/comments/1d3spoc/error_getting_submitted_with_mobile_useragent/