GamesIndustry.biz has some analysis of new game releases on Steam for 2019 based on data from sleuthy website SteamSpy. While more games came out on Valve's store/service last year than in 2018, they report a "major slowdown" of new releases. They call this a change of pace, as they note the rate of new releases increased "rapidly" each year starting in 2014. They also report the number of new games owned is down significantly year on year, but caution that this is not based on sales, just copies owned. Here are some figures:

Last year 8,415 games arrived on Steam, compared to 8,167 in 2018. With the number of games released on Steam rapidly increasing since 2014, this represents a notable change in pace.

The number of games released on the platform saw its first major leap in 2014, increasing to 1,609 from 557 the year prior. Since then, the number of games releasing on Steam has increased to nearly 2,000 a year.

The effect of this slowdown is seen throughout the data for 2019, most notably with the number of copies owned falling from 242 million in 2018 to 127 million in 2019.

SteamSpy is unable to provide precise sales data, but the number of copies owned "refers to total copies of games released this year acquired by users for the lifetime. Includes copies sold, downloaded for free and given out."