Let's hope it actually reads the available combinations from ~/.XCompose. I am pretty sure that this was not the case (at least at some point in the past).
GNOME 3.38 allows adding custom sequences using the .Xcompose file. The only thing that doesn't work are include directives. I'd imagine it will be the same with GNOME 40.
If you are a Romance language speaker you get tired of the local layout fast, specially for programming.
A lot of people use the US+compose key for accents/ñ.