I saw this note on the jisho.org page for 遊ぶ:
Other forms 遊ぶ 【あすぶ】
Notes あすぶ: 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, あすぶ is just a phonetic shift (音変化) of あそぶ. (source) From this description, it seems like あそぶ is the original spelling, and あすぶ is the derivative... so it doesn't seem like あすぶ is an old form of the word.
あすぶ is also listed in some dictionaries as part of regional dialects. For example, it's listed as ibaraki-ben (in 茨城弁大辞典), the agatsuma dialect (in あがつま語), and koushuu-ben (in 甲州弁辞典). (source)
In this hinative thread, a native speaker says that they never heard of あすぶ. All things considered, it seems like this pronunciation should be avoided for general-purpose conversation.