In 1987, Canon introduced the EOS system with the EF mount, which superseded the previous FD mount. The distance between the film and the flange of the mount was increased in the EF system, which meant that you couldn't put a simple adapter on the EF mount to take the older FD lenses and have it focus at infinity - the FD lens would be too far away and it would need some correcting lenses in between to get the FD lens to focus further back. The RF sensor to flange distance is much less than the sensor to EF flange, which means you can easily fit an adapter on the RF to take an EF lens and for it to be placed at the correct distance away from the sensor to focus. So, in 1987, Canon rendered all of its existing lenses obsolete. This time around, all existing older EF lenses are forwards compatible.