Resolved: Now the tag name is function-and-relation-composition - the name clearly indicates that it is both for composition of functions and relations.
I propose creating relation-composition tag and making it a synonym of function-composition. I think that if composition of functions is important enough to have its own tag, then so is composition of relations. But it would probably be better to have both topics under the same tag.
We definitely have some questions about composition of relations. This tag could make finding such questions easier.
It seems that currently some users are using function-composition tag for such questions. Seeing that there exists tag for composition of relations might encourage some users to use correct tags.