Backfilling is not a general fedi issue, you can already backfill posts manually by copy pasting the URL of a missing post into the search box on your server.
It should be possible to automate this process.
As for the age of issues, the follow hashtags issue was opened in 2017 and has just been added to Mastodon (you can try it already on mastodon.online and it will be in next general release). Old issue doesn't mean "won't fix".