Again, you're becoming confused here and assuming that I think all Starfleet captains should be able to recall hundreds of years of alien encounters from memory.Even if it weren't classified, chances are Picard hadn't read it. He can't read everything, and in a crisis situation, I can't see him running off to google 'robot space zombies' just in case someones blogged it...
I don't.
I think the unfathomably powerful computers used by Starfleet, continually accessed by Data who was linked into them, should easily be able to throw up information about similar past events when faced with them further down the line.
Thus when a Starfleet ship is presented with a vessel that is a mishmash of bits that regenerates when damaged, or cybernetic aliens with personal shields that quickly adapt to block your weapons and so on, I don't see how it is unreasonable that these phenomenal computer systems aren't instantly analysing records to see if anything similar has ever happened and find that yes, it happened on Earth and was encountered by the uber-famous, everyone's childhood hero Jonathan Archer and his equally uber-famous, still pride of place in a 24th century museum, Enterprise NX01.
If that information wasn't available for Data to access, then why not? Either it was lost or classified.
We've done the lost bit - the idea that 22nd through 24th century computer storage would be such that what happened in Regeneration would be consigned to a single file in a single location that was then blown up during the Romulan War is beyond laughable.
So if it was indeed classified, why classify it? People are continually claiming it is reasonable that Starfleet would classify the incident without explaining what purpose that would serve.