Profanity seems to do it. Even in a positive context. I have had it happen recently where I added a comment but it never made it into the comments. There was no error.

There may have been the word shit in my comment. But it was polite and constructive. Meh. Makes me not want to bother and I don't.

There is no first amendment right to have your comment visible on Youtube. In fact, Youtube has the first amendment right to show whatever comments they want. They could even have fake accounts that post positive comments and upvotes, and charge users money for artificial engagement. Google executives could decide that they wanted to support literal Nazis, and promote their videos, add fake likes and engagements, and do the opposite to any opposing political views. This would all be perfectly legal.

We have restricted the constitutional rights of corporations in other cases e.g. utilities, though. And we can do it again.

Because the Constitution and the Bill of Rights are a governmental founding document and only apply to the government. Alphabet is not part of the government, and therefore, it does not apply.

Discrimination by companies toward the public is covered mainly by Civil Rights Act (or the Americans with Disabilities Act in the case of discrimination based on disability).

A first amendment case based on what? They're not the government.