There are no absolutes in writing, but this advice, like a lot of writing advice, is better perceived as "editing advice".

This is sort of like the common advice to avoid "-ly" words, because more often than not there will be another option that will provide a stronger sense of detail and your own personal voice. These are elements of tone that will help your piece feel less generic to readers.

Going forward, write your first draft as it comes out of your mind, and that includes using "to be/is" as often as you just happen to. But when you go back through the piece, identify these places and do your best to find something stronger. There will be times when it is okay to leave the phrase or sentence as you first wrote it, but it's a good idea to at least attempt to find something better.

The reason that editors and professional writers treat this stuff so egregiously when talking to younger writers is that writers with less experience tend to have a habit of just trying to do the easy thing and go with what they first wrote. Also, when translating something we see clearly in our own minds, it is very common for writers to overlook details that seem obvious and automatic, but that aren't to our audience.

In the end, these kind of matters aren't sins. You aren't failing as a writer to use "to be" at times, but these spots in your piece are definitely opportunities to make it stronger.