Russia, has developed many rocket engine concepts over the years and its noticeable that they already tried liquid methane as a propellant back in the 90s. How many of their cryogenic engines had a liquid methane variant? Have any other countries also developed similar capabilities and tried liquid methane in the past?
According to some old usenet postings by Henry Spencer now on yarchive, the RL10 was experimentally used with methane:
"CH4/O2 offers the second highest Isp (385 s)... A significant disadvantage of CH4/O2 is that no flight-rated engine is currently available; development of one based on Pratt and Whitney RL-10 engine technology would probably take three years and cost on the order of $30 million."
One reason to be reasonably confident in this low number is that the RL10 has already been run on LOX/CH4, with only minor modifications. That was done only experimentally, and you'd need a bit more work to certify that configuration for production use, but it shouldn't be hard.
According to Clark's Ignition! (1972), German rocket pioneer Johannes Winkler experimented with a methane/LOX engine in 1930. Clark also mentions with some horror a 1957 proposal to use pre-mixed methane/LOX as a monopropellant. ("JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it.") Clark also mentions using methane with other oxidizers:
Rocketdyne, and TRW, with NASA contracts, as well as NASA itself, have concentrated their efforts on OF2 and the light hydrocarbons: methane, ethane, propane, 1-butene, and assorted mixtures of these. (In most of their motor work, they used a mixture of oxygen and fluorine as a reasonably inexpensive surrogate for OF2.)
Spencer also mentions the RL10 having been fired with FLOX (fluorine-LOX mixture), so it seems probable that Clark is talking about RL10 here as well.