I apologize for a delay but I had no computer access for a while… I’m not sure what does it mean, it could apply to older or still manufactured by some companies electric water heaters that have an opening (apparently they did). Maybe they were using the same bottom plates as the gas water heater has for electric water heater assemblies.
One possible reason is that they try to prevent a possibility of combustible vapor (e.g. from gasoline) from penetrating enclosure and igniting when the heating element’s relay turns ON or / and OFF.
Is that requirement published on the Internet? Do you have a link to it?