I have tested senna and so far she seems a bit on the strong side as a support. However ive noticed that her adc presence is somewhat lackluster, mostly due to the passive similar to Yasuo that reduces her crit damage. The intent is obvious, it is a method of making her support iteration less oppressive mid/late game. While it is a good measure of preventing support senna abuse, it makes adc senna pretty weak. I would propose instead of linking senna's instead of linking senna's crit overcap to lifesteal, instead give it a weak conversion to critical damage allowing Sennas who build multiple crit items the opportunity to deal damage similar to other adcs. This would make it so that sennas who prioritize crit can negate the debuff, and lategame senna adc and potentially gain more damage on her autos in a similar playstyle to Jhin.