I've been an audio engineer since 2006 and I've worked aaaaall sorts of jobs, from mixing underground indie bands to recording Grammy-nominated artists and I've never seen a single professional audio engineer be scared about the amount
they're boosting or cutting.
EQ is just a tool and you should use it to get the results you want.
The worst thing is when students are so indoctrinated with garbage advice like this that they get scared of their plug-ins.
These plug-ins go up to 11 for a reason. The only way to learn is to twist the knob to its breaking point.
You're not gonna ruin your recording.
If you're mixing in the box, you're only a "UNDO" function away from fixing anything.
Subtractive EQ is a great method for getting rid of annoying frequencies you don't want, but it's literally ONLY HALF OF THE EQUATION!
Subtractive EQ helps you get rid of all the crap you don't want so that you can add as much candy as you'd like!
It's stupid to fear the boost knob, and if you're taking advice from people who say stupid shit like that on the internet, you need to find better mentors.
You can boost as much as you'd like if it helps your instrument cut through the mix.
This "no-boosting" garbage WILL stand in the way of your mixes sounding better, which is why my Audio Issues EQ has an entire module dedicated to boosting the frequencies that enhance your mix.