MMO RPG anime are great! Is this a new trend, or are there older anime in this genre? I got hooked on the lost-in-the-MMO genre with SAO (Sword Art Online), but I didn't even know it was a genre until a few weeks ago.  I caught up on Log Horizon, and now I'm watching Overlord.  Obviously, MMOs are a relatively new phenomenon, but approximately how old is this genre of anime?

I am curious because I'm about to go looking for more titles and I'd like to know how far back to search and/or when to stop looking.
The earliest show I can think of where a character is trapped inside a video game is .Hack//Sign, which aired in 2002.

However, this isn't purely an anime feature - I read many books with similar situations, some dating before 2002. Example 2002, Example 1998

Presumably this genre is a type of mutation of the 'Trapped in the TV' trope.

It's an easy way to legitimize a fantasy world for an author, so it's become a common staple in teen/young fiction.

Whilst not exactly the same, there are earlier media like The 1980's Dungeons &amp; Dragons cartoon that have characters magically transported to a fantasy world. Or even books like Type Writer in the Sky (1940s) in which characters are transported to a foreign land through the pages of a novel. It's simply modernization of this trope.

So, to answer your question - 2000s and onwards is a good starting point, but you might find similar shows with a slightly different approach earlier than that if you want.