As of this writing,
Shohei Ohtani is slugging .700 on the season. In no way whatsoever do I think Ohtani will slug .700 for the entire season. But if he does, it would put him in historical company.
Here is the list of players who slugged .700 or better in a season (after WW2, since that's a somewhat fair break point):
Barry Bonds 2004
Barry Bonds 2003
Barry Bonds 2002
Barry Bonds 2001
Sammy Sosa 2001
Larry Walker 1999
Larry Walker 1997
Mark McGwire 1998
Mark McGwire 1996
Jeff Bagwell 1994
Frank Thomas 1994
Ted Williams 1957
Mickey Mantle 1956
Stan Musial 1948
Bonds, Sosa, McGwire and Bagwell all have very strong connections to being on the juice. Probably Larry Walker, too.
I'd be confident in saying that the only guys to slug .700 legitimately in the last 70 years are Williams, Mantle, Musial and Thomas -- and Thomas did his in a strike-shortened season.
If Ohtani can keep this up, it will be a miracle.
(for the record, prior to WW2 the list of players over .700 are Ted Williams, Jimmie Foxx, Babe Ruth, Lou Gehrig, Hack Wilson and Rogers Hornsby...but that's effectively a different game back then)