Why is あすぶ an "out-dated or obsolete" kana usage for 遊ぶ? I saw this note on the jisho.org page for : > Other forms > > Notes : Out-dated or obsolete kana usage. And I'm curious what this means - did it used to be read this way? Or do some people still read it this way? What happened?

According to many dictionaries, is just a phonetic shift () of . (source) From this description, it seems like is the original spelling, and is the derivative... so it doesn't seem like is an old form of the word.

is also listed in some dictionaries as part of regional dialects. For example, it's listed as ibaraki-ben (in ), the agatsuma dialect (in ), and koushuu-ben (in ). (source)

In this hinative thread, a native speaker says that they never heard of . All things considered, it seems like this pronunciation should be avoided for general-purpose conversation.