For
example, the semicolon (";") and equals ("=") reserved characters are
often used to delimit parameters and parameter values applicable to
that segment. The comma (",") reserved character is often used for
similar purposes. For example, one URI producer might use a segment
such as "name;v=1.1" to indicate a reference to version 1.1 of
"name", whereas another might use a segment such as "name,1.1" to
indicate the same
There’s no thought given as far as I can see to the relative URL concepts this matrix proposal covers, but the idea of adding key=value data in this format is supported by the RFC, and indeed in a way that supports further adding sub resources below the current one.
Literally every ad you see is a business deciding ‘instead of showing you what you came here to see, here’s something else, which I am showing you only to benefit my business.’
I’m not sure what limiting principle you can apply to limit the ability of businesses to show users other things the user might be interested in that wouldn’t also basically ban advertising too.