And perhaps stating the obvious, if you collected all the EP needed for Platino Prime from 1 April 2026 to 1 June 2026 (for example) you would enjoy that status until 31 March 2029? With no further points earning you would have no status from 1 April 2029 (I.e. there is no soft landing)?
Until 10 April 2029 including the generous 10 day grace period.
Iberia Club doesn't officially have soft landings but in reality they usually do, based on experience from Iberia Plus. I'm currently Oro after having been Platino (under the old system) until one year ago, and having flown almost nothing in my last year of Platino status. I would expect Platino Prime to soft land to Oro (because soft landing to regular Platino seems way too generous). I haven't requalified as Oro so we'll soon see if I get a year of Plata or not. So in your example, Oro until 10 April 2030 seems likely even if you fly very little 1 Apr 2028-31 Mar 2029.