Here's my argument:

In practice many GET requests don't adhere to this spec. For example, when you load a page, your "view" generally changes lots of things on the backend. Those changes come back to you in ways too: for example, consider view counts on Youtube videos or X posts.

These are just conventions, one can pretty much do whatever they want in their applications. At the same time convention has its own advantages (most of the times, think about code maintenance). Its still along the excepted line as long as the mutations are side effects of the GET request. Somewhere down the line the intent is to provide a separation to easily understand the systems.