Precursors to Isekai 異世界
https://en.m.wikipedia.org/wiki/Isekai
☞ Connecticut Yankee in King Arthur's Court, Mark Twain, 1889
https://en.m.wikipedia.org/wiki/A_Connecticut_Yankee_in_King_Arthur%27s_Court
☞ The Magician's Nephew, C. S. Lewis, 1955
https://en.m.wikipedia.org/wiki/The_Magician%27s_Nephew
☞ Three Hearts and Three Lions, Poul Anderson, 1961
https://en.m.wikipedia.org/wiki/Three_Hearts_and_Three_Lions?searchToken=8ddwrnqif0gurxhg9godp396z
☞ The Twilight Zone, Rod Serling, 1959
https://en.m.wikipedia.org/wiki/The_Twilight_Zone_(1959_TV_series)
☞ Glory Road, Robert A. Heinlein, 1963
https://en.m.wikipedia.org/wiki/Glory_Road
☞ Lord Kalvan of Otherwhen, H. Beam Piper, 1965
https://en.m.wikipedia.org/wiki/Lord_Kalvan_of_Otherwhen
☞ Chronicles of Amber, Roger Zelazny, 1970
https://en.m.wikipedia.org/wiki/Glory_Road
☞ Spellsinger, Alan Dean Foster, 1983
https://en.m.wikipedia.org/wiki/Spellsinger
I'm sure I'm missing some seminal works which could qualify. Also The Twilight Zone only deserves honorable mention as many of its episodes wouldn't qualify as isekai. However, enough of its stories are so influential, and develop themes which could implicitly be isekai (in the sense that the protagonists have slipped into these "other worlds" without any exposition on how it occurred) that I felt compelled to include it.
But we might make similar arguments for The Outer Limits, or even Night Gallery. Where do we draw the line?