As title says, I'm not sure if this is intentional or a bug but it would be nice to have the 'By Speed' % in the retimer respond to keyframing. Only the 'By Frame' option can be animated at present - v0.16010 (BETA)
Not being able to keyframe the speed % value is indeed intentional and noted in the docs. This may change in the future but for now if you need variable playback speed you have to use frame mode.
