It was/is but it's always been kind of "look the other way" type of deal unless it was painfully obvious, like guys with huge wads of pine tar on their hat etc.
Most hitters seem to be mostly OK with pitchers using a little something (pine tar, sunscreen/rosin mix, etc.) to help increase their grip so they feel safer in the batter's box. Pete Alonso, the 2019 Rookie of the Year, came out and said a week or two ago "Because I go in the box every single day and I see guys throwing harder and harder. I don't want 99 slipping out of someone's hand because they didn't have enough feel for it."
It seems to be the guys using spider tack or similar stuff to get insane spin rates that guys have a bigger issue with. I mean for god sake the MLB keeps a rosin bag behind the mound every game. Rosin alone doesn't do a lot unless you really really grind your fingers together to get some friction and heat it up, but that takes a minute or two and with the pace of play stuff MLB is so worried about, isn't really a viable option to take a minute between every hitter to get more rosin grip. The faster way to get rosin to work is to mix it with some form of liquid (sweat, spit, sunscreen, etc.) Mixing those creates a similar grip to what rosin alone does once you get it "heated up" for lack of a better term.
The MLB constantly tinkers with the baseball during the offseason. Seemingly every year. Maybe if the MLB would stop tinkering with the ball, pitchers wouldn't feel the need to find concoctions to help grip the ball better.
If you've ever tried to play catch with a real MLB game ball with the lower seams and polished leather, it is definitely more slick than the game ball your kids use or even your high school/college kids use. And the seams on those balls are significantly higher, too.