It’s not fully backwards compatible but is “builds upon SVG 1.1 Second Edition”.
So it won’t be a problem for most SVG elements but there still is a few breaking changes SVG 2 breaking changes · w3c/svgwg Wiki · GitHub
(You can also see the new features here SVG 2 new features · w3c/svgwg Wiki · GitHub for a more readable version of Appendix K)
My concern was raised by the <use> element <use> - SVG: Scalable Vector Graphics | MDN and its older xlink:href with the associated namespace xlink: that might not be supported at some point by browsers. But is still heavily used…
In addition, the status of this W3C standard is very unclear to me as there was no significant progress/interest since 2018, but still the browsers implemented it ("svg use: href" | Can I use... Support tables for HTML5, CSS3, etc)
So again, composer responsibility ?
Me too 
And thanks for all the details/explanations !