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Since its original release on the Neo Geo AES/MVS in 1996, the first entry in the Metal Slug series has been reissued on many different platforms over the years.
Not only did it get a set of Japan-exclusive ports for the Sony PlayStation and Sega Saturn in the late '90s, but it has also found its way into a ton of compilations and collections, and has been released digitally on pretty much everything from smartphones to modern consoles (like the Nintendo Switch, Xbox One, and PS4) in the years since. One version of the game I'm guessing you probably aren't all that familiar with is the Vodafone J-Sky/Vodafone Live! version, which released in Japan back in 2003.
Looking online, there is very little information about the game in either Japanese or English, with the best we can dig up in terms of press coverage being an itmedia article, covering its appearance at the Tokyo Game Show, where it was described as "a port of the popular NEOGEO title" with scaled-down characters to fit the mobile screens, but roughly the same content as the Neo Geo version. The Japanese Wiki doesn't even include it in its list of "other versions" of the game, suggesting it has pretty much been forgotten by players in the time since.
Despite that, though, remarkably, it appears you can actually now play the game for yourself, thanks to the recent efforts of Cuebus, one of the members of the Keitai Wiki Discord preservation group, who has rescued the title from vanishing forever.
[Note: the below clip features some flashing lights after the mission select, so photosensitive readers may want to skip ahead to the actual gameplay]
The excellent news came to our attention, thanks to a social media post from RockmanCosmo, who labelled the title "a simplified version of the original Metal Slug game" and praised its small sprites as "really cute." He shared some footage of the game, which showcased the revised level layouts and audio, and the Newgrounds musician Chromadeline replied that it was "surprisingly accurate and well done," despite the platform.
You can watch some more footage of the game in action below, from the YouTuber Mr^Burns. The game, meanwhile, is available to play now on PC using the Keitai World Launcher, if you want to give it a go yourself.