Sivir also can't spellshield Ekko's W or Nasus' E

Sivir can not spelshield Nasus' E initial damage at all, it does not break the shield. Sivir can not spellshield the slow from Ekko's W at all (also does not break the shield), and if Ekko would have stunned, it eats up the spellshield and there is no stun, but she is still slowed. There might be more, since these two abilities are really similar.

Sometimes she can not spellshield Varus Q, but you must already be aware of this(there were Riot responses on the Reddit thread).