Why do some YouTube video channels say that they are not allowed to say the word "Covid"?

Probably has to do with demonetisation (threats).

To combat misinformation in regards to coronavirus (motivated by the desire to not lose advertisers), youtube bots demonetise videos that use the word.

YouTube also took down a video of a liberal political channel making fun of Trump supporters. They said mockingly in a country accent “Trump done won the election!”

The YouTube algorithm really sucks.

It is an algorithm issue, but it is also mentioned to remind people that they can’t always speak freely, be it covid or other topics.