It looks like a next-generation Lexus IS is in development, and set to debut sometime in 2020 for the 2021 model year.
It has been rumored that both the IS and GS would be unceremoniously axed in an effort to feed America's insatiable appetite for crossovers.
However, according to a post on Lexus Enthusiast, it appears that Lexus does have plans beyond the current generation for the IS. The post details Lexus's plans through 2020, including the production plans for the 2019 ES, and mentions an upcoming redesign for the 2021 NX.
It goes on to say that the fourth-generation Lexus IS has been in development stages since 2015, with "Job #1" (completion of the first production example) set for July 2020. "Likely a design freeze will happen early in 2018, to allow 27-30 months of lead time, poster Carmaker1 says. "A final styling proposal for what is nicknamed as the '4IS', will be chosen this year before that point of development is reached in 2018."
Motor1 says that car could have a twin-turbo V6, the same unit found in the new Lexus LS 500, producing around 400 horsepower. The report is based on a story in Driving Enthusiast (no relation to Lexus Enthusiast), but that story does little to substantiate its claims, so take it with a grain of a salt.
In the meantime, the Lexus Enthusiast post says that "a minor secondary update might occur to the current IS for model year 2019 in late 2018, tiding the current model along until 2020.?" before discussing RX, GX, and LX models. Notably absent was any mention of the GS, further leading leading one to think that the GS is destined for the chopping block. But it seems the IS will live on.