per Yahoo Help Team -- "Regrettably, the 'Keyword' option is no longer available in the advanced search for the new Yahoo Mail. We apologize that our current articles were misleading regarding this feature. I can see how that'd be useful, though".

In New Yahoo Mail in the top search bar, click the Advanced Search icon. The search criteria of Keyword is NOT listed per Yahoo help screen. Hence I cannot do Search for multiple criteria per Yahoo help screen -- example ALONG with sender name, date range ++ Keyword as listed on Yahoo help page at
https://help.yahoo.com/kb/SLN36721.html

In my OLD Yahoo email, BLUE button for Advanced Search by keyword is missing. This existed few days ago,

In another Yahoo email account the Blue Advanced Search button is visible on right corner of Search button. Once click >> Keyword search is available >> with other criteria of sender, to, subject, date range, contains (photo, attachment, calendar invite).

Till issues are fixed in new Yahoo email, the Classic Yahoo option should not be removed.
Also if users are paying for other Yahoo features they must be waived till ALL OLD features are available in New Yahoo.